This is a very useful website for calculating the true cost of credit. It's an American site so you will have to read $ as £.
Loan calculator (includes amortization schedule)
? How's your credit?
? Find the best credit card for you
? Debt Pay-Down Adviser: What's the fastest and cheapest way to pay down your debts?
? See your FICO Score Range -- Free
? Calculate the real cost of your debt
? What will it take to pay off your credit card?
? The true cost of paying the minimum
http://www.bankrate.com/brm/calculators/credit-cards.asp