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Her secret weapon is Helen Rhiannon, a fashion designer from Swansea. Helen Rhiannon launched her own clothing label in 2005 and counts Katherine Jenkins among her celebrity clients.

She agreed to lend a hand to our teenage fashionistas, but first Rachel wanted to know more about them and their prom plans.

They told her, "We're so looking forward to it, it will be a really nice way to say goodbye. What you wear is so important, you can't wear the same thing as someone else."

So while Rachel left Helen Rhiannon to work out what shapes and colours suited the shoppers, she discovered from Harriet's mum Carolyn that the average spend on a prom outfit can top £200.

Carolyn says, "By the time you take into account the dress, then they'll want matching shoes and a handbag, new jewellery, you're talking a small fortune."

So Rachel set the challenge - shopping for all their outfits for the price they'd normally pay for one - a £200 budget for four sets of tip-top party togs.

Rachel took them all to Aberystwyth. Because there are so many students there it's likely the charity shops would stock a good selection of clothes to suit our teenagers - fun and funky threads they could really shine in!

They were given three hours to hit the shops and get back to Helen Rhiannon who was on hand with her sewing machine and accessories to alter and adapt anything they found which might be suitable.

Buying and modifying clothes from charity shops is becoming more and more trendy. With an emphasis on environmental awareness and sustainability, there's less stigma about buying second-hand and more kudos for the kitsch and quirky finds that come under the banner of vintage.

It's good for the charities too; every year charity shops raise more than £110 million across the UK for the causes they support.

If you're looking for a fashionable bargain, here are Helen Rhiannon's tips:

"If you find something fantastic straight off then stick with it and maybe consider altering it, maybe making it a little shorter, or you can just add little sparkly diamantes and like ribbon for 50p a metre in your haberdashers which will make it different to when you first have it.

"If you're clever you can find accessories and things that will jazz up outfits and change them."

So how did our shoppers do? Well, Alison looked awesome in a £25 bargain from Tenovus  and Harriet could charm a catwalk in a black satin number from Save the Children.

Dafydd was set to dazzle in a classic dinner jacket, a snip at £17.50, coupled with a £4 shirt from Cancer Research UK, and cute as a button, Emma strutted her stuff in a classic prom dress from Barnardos, embellished with ribbon and a corsage by Helen Rhiannon.

It was a challenge but they looked stunning and the total spent was just over £115 - well within budget! Result!