have I got any credit on my account from a previous gift card that I redeemed?

We are fellow users here on these forums, we won't know.

My account's balance shows at the top left of the store's page on my computer's iTunes, and by logging in to view my account via the Store > View Account menu option. On my devices it shows at the bottom of, for example, the Featured tab in the App Store app under my account id (or you can view your account via Settings > iTunes & App Stores and it should show on the screen that you are taken to)