I think it's automatically applied to the credit on your account. When I went to the 'Make a Payment' page my outstanding balance was only £35. Also I'm 27 have zero credit history and they wouldn't accept me lol. I had to use my Mum's details, with her permission of course.
I ordered it, says total £40.95, guessing because of the games I ordered. Balance is £40.95 on the website.
Not going to complain about them giving me a credit account, lets me get something on the credit history