I have £5.02 left on an App Store card and wish to buy an app costing £1.99 but it asks for a credit/debit card number. How do I get it to come off my credit?
Likely the store is just trying to make sure you are the actual person with the account, and so it is asking for verification information rather than wanting to charge your credit card. If you have already entered credit card info in the past, re-entering it will not change any of Apple's data, but will verifiy that you are the same person who set up the account.
I beleive you can also hit the "none" button beside all the credit card icons if you do not have a credit card on file and are just relying on gift card credits.
from: Apple Support
The entire amount of a gift card is credited to your iTunes Store account when you redeem it, so you don't need to enter your gift card number or gift code each time you want to buy something.
View your credit balance
After you redeem iTunes Gift Cards or iTunes Gifts, the amount will appear in your iTunes Store account. Each time you purchase content, iTunes will deduct funds from the credit until it's depleted. If you have a credit balance from a gift, it will appear in the iTunes Store, App Store, and iBooks Store on your iOS devices and in iTunes, iBooks for Mac, and the Mac App Store.
View your balance on your iPhone, iPad, or iPod touch
Tap the iTunes Store, App Store, or iBooks. Scroll to the bottom of the Featured tab. Sign in with your Apple ID. If you have a credit balance, you'll see it below your Apple ID.
It seems unlikely to me you have £5.02 left to redeem from your card, since the entire amount of a card gets credited the first time you redeem it. You should check as stated above if you have enough funds in your account (below your apple ID). If not, that's why you keep being asked to add a new credit card or to redeem a new gift card.