"Only" £300 extra?? That sounds crazy. What was the interest rate?
Get the Post Office credit card and pay it off over 27 months with 0% interest.
http://www.moneysavingexpert.com/credit-cards/best-0-credit-cards#postoffice
However considering you deem £300 interest spent on a PC reasonable I might suggest that credit is not the best option for you.
At checkout change payment method, it's in there