The ballad of St Kilda

We've got another couple of hours to go until we reach our destination: St Kilda. The most remote part of the British Isles, an indelible part of Scotland's romantic history, the archipelago sits alone in the Atlantic, 41 miles west of Benbecula in the Outer Hebrides. It may welcome as many as 2,000 day visitors each year, but only a handful of people get the opportunity to stay on the island. I'm one of them.

Bumping across the water, 11 of us are heading to the cluster of islands known as St Kilda, and most are trying to keep hold of our hastily eaten breakfast. I'm hitching a ride with a working party made up of experts from the National Trust for Scotland (NTS), which is responsible for the archipelago, and the Royal Commission on Ancient and Historical Monuments (RCAHMS), as well as five volunteers. As the archaeologists and surveyors continue their work mapping Hirta, the biggest island, the volunteers will carry out general maintenance work – painting roofs and clearing guttering, fixing a broken window and clearing drains. It's not glamorous, and they've paid for the privilege, but in exchange they'll get to spend two weeks on the island. Priceless.

St Kilda is one of only a few places in the world with Dual World Heritage status, recognising both its natural and cultural significance. Part of the island's appeal comes from the fact that it was evacuated of its remaining indigenous population in 1930. Struggling to survive, their numbers had dwindled and life on the island was becoming increasingly harsh. The place, and the lost lifestyle of the islanders – living in isolation, sharing their catch of seabirds and eggs which they clambered down the steep cliff faces to catch, ropes tied around their chests – intrigues us today just as it did the Victorian tourists who travelled to the island on steamers, to marvel at the St Kildans and buy their blown eggs or hand-spun wool as souvenirs.

Of the group I'm travelling with, only two have never been to the islands before. It seems once you've visited Kilda, as they call it, you will want to return.

I've got three days on the island to see at first hand the work that's being undertaken by the NTS and RCAHMS to survey Hirta, work that will be used to create the most detailed and accurate map of the islands ever completed. This will, in turn, help guide conservation work. A three-year project which began in 2007, it owes much to the groundbreaking work of archaeologist Dr Mary Harman, another St Kilda devotee. During the 1970s and 80s Harman created, largely on her own, the first detailed map of the islands. Scrambling down steep banks and vertiginous cliffs, she plotted the island's 1,400 or so "cleits", the small stone structures topped with turf that pepper Hirta, and can also be seen on the surrounding islands of Dun, Soay and Boreray.

The stone structures add to St Kilda's lore (no one is quite sure what they were used for), as well as adding to its unique landscape. Harman is a hero to all the teams working on the island now and it's her field work, continued by a team from RCAHMS in the 1980s, which is now being combined with aerial photography and centimetre-accurate, satellite survey technology to plot all the known sites on Hirta as well as to locate new ones – ruins, the remains of structures, walls and even areas that have been farmed. Once the map is complete, as well as providing a unique and valuable record, it will help the NTS protect and maintain the remote landscape. Many of the images and documents will also be available to the public through RCAHMS online databases.

Suddenly, out of the greyness, St Kilda appears. Sheer cliffs rise from the sea, the air is speckled with seabirds – kittiwakes and gannets – swirling around the rock faces. The islands are home to more than half a million breeding seabirds – the largest colony in Europe – and the air is filled with their calls. As I'll discover, the legends and stories surrounding St Kilda far exceed what the islands themselves deliver, but at the same time they fail to capture its unique atmosphere. I can't think of anywhere quite like it.

DAY ONE

The boat moored in the bay, I push a wheelbarrow filled with supplies up a wet, grassy verge, my legs still shaking from the journey. Soay sheep, the ancient fluffy brown breed unique to the island, glance disinterestedly as we straggle past. The "village" on Hirta consists of one street. Of the cottages which line the street (built in 1860 after a visiting minister was appalled to see the islanders still shared their blackhouses with cattle), five have been refurbished and provide accommodation for working parties. There's a communal kitchen and a small museum for day visitors to the island, as well as a store and two dorms. The others are roofless shells, but in the fireplace of each sits a slate painted with the name of the family who lived there before evacuation. It's a sight that's eerie and comforting at the same time.

In the women's cottage, five single beds dot the wood-clad room. It smells musty and damp (and at night you can hear the scratching of mice), but filled with sleeping bags and kit, with boots and waterproofs hung on the rail around the room, it looks cosy.

Back outside, I sit on a small stone dyke to take in the view. It might look exactly as it did hundreds of years ago, apart from the MoD base, now run by defence technology company QinetiQ, down by the shoreline. Images of St Kilda are usually artfully cropped to remove the boxy buildings, the electricity generators and the helipad. They might not be part of the romantic view of the island but since the base (which is used as a radar tracking station for the missile range on Benbecula) was built in 1957, they are part of what the island is about. The Land Rover that zooms up the steep road twice a day as the receiver is checked is as much part of the island soundscape as the waves that lap on the stony beach.

As the sun breaks through the clouds, George Geddes, one of the National Trust's archaeologists, who's been living on the island for six months, guides me round the village area, showing me the traces of a head dyke which defined the area used for living and farming by the islanders and, of course, the cleits.

Everywhere you look – up the steep slopes of Oiseval and Conachair, the hills which loom over Village Bay – there are cleits of different sizes. Much of the lore of St Kilda is built around these unique structures, which look something like igloos, but built with round-edged Dolomite stones and topped with turf. Built from the inside with the door upslope, the floor of the oblong rooms sloping down, it's most likely that they were used as stores for keeping seabirds and eggs or for drying turf and peat, but it only adds to the romance that no one really knows. Most of the ones I pop my head into are full of sheep bones and fleece. Some of them have additional cells joined to the main structure, some are large enough for ten people to stand inside, some so small that only a child could squeeze in. On the higher slopes, they look like smudges of stone or little bird boxes, gently sliding down the hill as time and the weather take their toll.

Clambering up the slopes behind the village, I stand at the top of vertiginous cliffs. The sky is blue and kittiwakes glide and circle before landing in their cliff-face nests. Straight ahead, across a streak of the Atlantic that sparkles in the sunlight, stand Stac an Armin and Stac Lee, needlepoints of rock that rise high and proud, beside the island of Boreray. Seven miles from Hirta, on a day like this the rocky outcrops look benign, but for the St Kildans who travelled in small rowing boats to collect seabirds and their eggs, they could be treacherous. It wasn't unknown for hunting parties to be stranded by bad weather, forced to take shelter in the cleits until they could return safely. As I trample back down the hill Village Bay looks like a mouth gulping in the ocean.

As the first day passes, the huge skies have been grey and blue, cloud-filled and clear. As the light fades, I'm standing on the end of a stone pier and soon the darkness will be inky black. Shiny waves slap the spray-covered stone and the wind whips around. The only noise, other than the swirling water, is the quiet squeaking from inside a rough jute bag at my feet.

National Trust island ranger Bill Shaw gingerly lifts a puffling from inside the bag. Ruffled and grey, the scruffy bird looks fragile and small against the massive blackness of the sea and sky. The chick was rescued earlier in the evening, lying near one of the store buildings, stunned and immobile. At night there are only a few lights on Hirta's buildings, but they're enough to attract some of the young birds as they emerge from their burrows on the other side of the bay to take flight. This one's lucky. He'll get a second chance.

Wrapping my fingers around the downy, panic-filled body, the chick squirms and struggles, and tries to bite. My own nerves are almost as frayed, but with a firm grip, the count of three and a bit of luck, the puffling will be flying again, on his way out to sea. In recent years the number of birds breeding on St Kilda has dropped drastically. Colonies of puffins, kittiwakes, guillemots, fulmers and petrels have diminished. It means that every young bird is precious. If the chick squirming against my grip makes it, it will be as long as five years before it returns again, to breed.

Launching the bird up into the smothering darkness, instantly its wings beat frantically, fighting against the cold air and fast-approaching sea. Struggling to see the black body against the night sky, for a minute, it seems like it's failed, the chick plummeting into the water. A second later, as my eyes blur looking into the blackness, there it is, rising again. On its way.

DAY TWO

Standing in a shower cubicle, a plastic curtain blowing against my legs, a huge daddy long legs dead in the tray beside my feet, I'm reconsidering something I wrote a year ago in a context very different to this one. Having spent a week on a private island off the coast of Malaysia, full of wealthy tourists being pampered to within an inch of their lives, I said the best shower I'd ever had was one where, as the water spilled over me, I had a perfect and uninterrupted view across the Malacca Straits. There's no view here. I've just been on a seven-hour hike across Hirta, from Village Bay up over Conachair, along beneath a rocky brow, down through Gleann Mor, before finally ascending to the Cambir, the sheer, rocky edge of the island. It was a slog, my ankles are rubbed raw, my knees and feet aching from the rough ground, but as I lay on my stomach in the grass the final vista was breathtaking. Looking across to Soay, connected to Hirta by only two spikes of rock piercing the sea, it's an amazing sight. There in the scree is the faintest of outlines of a plane that crashed into the island, like a modern fossil on the surface of the rock. What makes it even more special is that not more than 50 people every year get to see this view. For day visitors, it's too far to walk in the time they've got on the island, and even for those lucky enough to stay, the weather isn't always kind enough to allow it. As the sun warms my back, the sound of bonksies (Great Skuas) soaring above, the odd wren chattering and even one wayward puffin which should've long left the island for its ocean home provide the soundtrack.

Hirta's landscape is littered with sites of archaeological interest. As well as cleits there are remnants of planticrues (enclosed beds for growing crops), sheilings (basic dwellings) and lazybeds (banks of ridges and furrows used for planting), scattered stones mark the remains of dykes dividing the grass covered slopes. As we walk through Gleann Mor towards the bay on the north side of the island, shining shards of fuselage from the Second World War plane that crashed here jut from beneath rocks and from under tangles of grass. Using their mobile equipment, members of the team digitally plot each one. They'll be added to the maps when the teams return to the mainland.

As we head back, over one of the grassy cliff edges, the team spots something that hasn't been noted before. To my untrained eyes it's barely more than a mound of stones of various sizes, but for the archaeologists it's a complex structure. From the scattered boulders they can see columns and evidence of more than one cell. Perhaps it was a bothy to offer shelter to hunting parties? Perhaps it was another store?

The discussion continues for the rest of the walk and later in The Puffin, the island pub and my final St Kildan landmark of the day. In a large hall, the bar is tucked in a corner. The music is eclectic and the walls are decked with signed sports shirts and flags from visiting boats. Hung high near the ceiling there are lifebuoys left as souvenirs. It's a pound a pint and there's a pool table and dartboard. Day visitors are not allowed in the bar, so as I drink my pint, I feel privileged indeed.

DAY THREE

Sitting on a rock on the tiny strip of sea-smoothed stones that edge Village Bay, the water looks green today and the sky even bigger than yesterday. As the waves lap the shore, it appears calm but in my line of vision there's evidence of how dangerous it can be. Lying foundered on the rocks is the Spinningdale, a trawler that ran aground last year. A salvage operation is planned but there's a wrangle about who will pay for it, so for the moment the boat will lie here like a sentry who has fallen asleep.

As sheep wander by and a pair of stints skip over the rocks, expertly dodging the waves, I head back to the village, to pick up my bags, only a little worried about how rough the sea looks. The stone path feels a little more familiar beneath my feet than when I pushed that wheelbarrow two days ago. On the stone dyke edging the path, lying lifeless, fluffy feathers bedraggled by the wind, is a puffling. Rucksack on my back, I wait for the boat with the day visitors heading back to Harris. This time there's no puffling to set free.

• For more information visit www.nts.org.uk and www.rcahms.gov.uk If you're interested in participating in a work party to St Kilda, apply at www.kilda.org.uk or send a stamped, self-addressed envelope (large letter) to St Kilda Working Party Coordinator, NTS, Balnain House, 40 Huntly Street, Inverness, IV3 5HR.