X Factor star looks for first boyfriend after losing 18 stones

But, as millions of television viewers and the hard-nosed judging panel would soon discover, there was more to the softly spoken singer – over 18 stones more – than it first appeared.

Judges Simon Cowell, Cheryl Cole, Louis Walsh and Danni Minogue stared open-mouthed as the Edinburgh call centre worker told them how she had shed a massive 18-and-a-half stones – the weight of a baby elephant – in just two years.

As the cameras zoomed in, she spoke movingly of how her new-found trim figure had finally given her the confidence to follow her dream to sing.

Lisa won the panel over with her performance of the Fleetwood Mac classic Songbird – but only after being gently turned down by Girls Aloud star Cheryl Cole and then begging notoriously difficult judge Simon Cowell to change his original decision and give her his vital vote.

His reluctant "yes" guaranteed her a place in the next stage of the fiercely contested talent show.

Today, 23-year old Lisa is back at home in Leith, exhausted after a whirlwind week of television appearances and magazine photoshoots, all kicked off by her remarkable performance on Saturday's X Factor.

But while she is remaining tight-lipped about whether she makes it through the challenging "boot camp" stage of the show – viewers will find out on Saturday – she has revealed that in spite of her massive achievement, the battle to achieve the perfect body still isn't over.

For despite shrinking from a whopping size 34 to a trim size ten – "I've even got a pair of trousers and a jumper that are a size eight," she proudly declares – after a lifetime struggle with the scales, Lisa now faces surgery to complete her transformation.

"I've still got a long way to go. When I first started slimming, I didn't think about the end result. I thought that would be me and I'd just be slim. But I still have to have surgery," she says.

"I'm waiting to have a tummy tuck done. I need to have surgery to remove the extra skin on my neck and my arms and my legs and I'll need a boob job. When you see my arms there is a lot of loose skin."

Pictures of how Lisa looked before she embarked on her healthy eating plan were passed around the X Factor judges before her performance. They showed a plump, smiling brunette in baggy clothes – barely recognisable as the fashionable slim blonde Duffy-lookalike in leggings and figure-skimming top standing before them.

"I wouldn't have been there if I hadn't lost weight," says Lisa, who now weighs 11 stones and was named 2007's Scottish Slimmer of the Year.

"I knew I could sing – I used to always be chosen to perform a solo piece in the school choir and I used to be in an operatic society in Kelso where I grew up.

"But I would never have had the confidence to go on the X Factor while I was so big.

"I decided last year after winning Scottish Slimmer of the Year that my next goal was to appear on X Factor – and now I've done it!"

Lisa had gradually piled on the weight since she was a child.

By the time she had decided to do something about it, she tipped the scales at a hefty 29-and-a-half stones.

"I weighed 10lbs when I was born, so I was big even then," she explains. "I was a tubby toddler, a fat child and a fatter teenager. My weight went up with my age: when I was 12, I was 12 stones, when I was 15, I weighed 15 stones.

"The doctors tried to blame my mum at one stage, but it wasn't her fault. She gave me healthy food to eat, but I'd go off and have crisps or snacks behind her back. I just loved food – I still do!"

Lisa tried to diet but nothing made any difference – mostly because she hadn't reached the stage of really wanting to put in the effort.

It took a dramatic television programme which featured a grossly-overweight man being winched out of his bedroom window to make her change.

"I was 21-years-old and almost 30 stones. Imagine if I hadn't started to diet. By now I'd be four, maybe five, stones heavier," she groans.

"I saw this man on television who was so fat he couldn't get out of bed. He was ill and paramedics had to knock down a wall and use a crane to get him out the house.

"I thought I didn't want that to happen to me."

She joined Scottish Slimmers in January 2006 and lost eight-and-a-half pounds in her first week.

"I was able to eat lots – I just had to make sure I was getting the right balance. It really was easy."

As the weight vanished, Lisa started to realise how miserable her size had been making her.

"I was always the bubbly overweight girl at school. If someone made fun of me I'd laugh along. I suppose you've got to learn to laugh at yourself.

"I did what everyone else did at school, PE, swimming and so on because I didn't want anyone to think I couldn't do it. So I ended up quite popular at school. Outside school people would look at me in the street or say things, but I didn't let it get to me.

"I can now go out clubbing or buy clothes in any shop I want, I realise how much happier I am."

Yet while she's achieved her X Factor dream – even if she lets slip that at boot camp she saw many singers who were more accomplished, suggesting she may not have reached the latter stages, Lisa still has another goal.

"I've never had a boyfriend," she giggles. "And I still don't have one. I can't get my head around the fact that I'm actually thin. I'm not 100 per cent confident with guys – if one starts to talk to me, it doesn't really click that they might be chatting me up. I'll get there eventually – but right now I'm still learning to live as a different person."