General Election 2010: 3:30am: When Cameron may know if he's next PM

WHEN the election was called on 6 April, the Electoral Reform Society issued a press release telling 25 million voters in 382 of Britain's 650 constituencies that their vote did not really count.

• A thoughtful Conservative Party leader David Cameron during his marathon round-Britain tour. Picture: Getty

The press release bemoaned the way the first-past-the-post system gave many MPs a job for life, including 36 of Scotland's 59 MPs.

In effect, this election will be decided in 268 marginal constituencies across the UK, which is where the three main party leaders Gordon Brown, David Cameron and Nick Clegg have concentrated their efforts.

In 1992, the early Conservative win in Basildon, Essex, set the tone for the night and John Major's victory rapidly became easy to predict.

In 2010, it is likely to be very much more difficult to forecast, with the polls suggesting a very close-run race, further muddled by the Liberal Democrats' surge in support following Nick Clegg's strong performances in the television debates. To make matters harder to predict, opinion polls have offered very different pictures of how the country will vote.

The closeness and unpredictable nature of the election mean that no single result like Basildon is likely to give an early indication. Most experts believe there will be widely different voting swings and patterns in different parts of the country and within regions.

The four main regional battlegrounds will be the West Midlands, the north-west of England, London and south-west England.

In the first three, Labour and the Conservatives will largely be looking for decisive victories in places such as Bolton and Birmingham. The results are likely to come in at about 3:30am, so this could be the point David Cameron knows if he has won.

The south-west of England is representative of the Tory fight with the Liberal Democrats, which to a certain degree is replicated in the south-east.

If Mr Cameron is to win he has to regain seats in Devon and Cornwall from the Lib Dems, but whereas these were bankers when he started in early April, they are far from certain now.

However, even though the campaign has been dominated by the television debates, which have threatened to squeeze out the smaller parties, this could also be a significant election for fringe breakthroughs.

In Barking and Stoke, the far-right British National Party is hoping to win seats, although in recent days the party's chances have appeared to falter.

In Brighton and Norwich, the Greens are putting up a strong challenge to finally get political representation in Westminster, while in Buckingham the Speaker John Bercow is facing a challenge from Ukip.

Scotland looks set to return predominantly Labour MPs again, despite the party's woes in England, but there are still interesting clashes, particularly in Edinburgh where four seats – South, South West, North and Leith, and East – look tight.

Tussles in Dundee, Livingston, the Western Isles and Ochil and South Perthshire will also show whether the SNP is in serious trouble or if it can revive some of the anti-Labour feeling it generated to win the Glasgow East by-election in 2008.

The Tories hope to come in from the cold in Scotland and win a significant number of 11 target seats, most interestingly East Renfrewshire, the seat of Scottish Secretary Jim Murphy.

There could be some high-profile casualties. Along with Mr Murphy, Chancellor Alistair Darling in Edinburgh South West is under threat, as is Children's Secretary Ed Balls in Morley and Outwood, and Culture Secretary Ben Bradshaw in Exeter. Tory Oliver Letwin is in danger of losing Dorset West.

While only 268 constituencies may be decisive, the closeness of this election means that for the first time in 18 years there is everything to play for.