kieronoddie Thank you for contacting the giffgaff Community.
Hopefully the following information will help you with your issue.
Airtime Credit was most likely used for Data! For example £10 Airtime Credit equates to a measly 200MB of Data at 5p Per MB with Airtime Credit, usually eaten within minutes or even seconds (even with just background Apps running), it's best when you top-up with Airtime credit to buy a Goodybag right away when you add the Airtime Credit and then get the free Calls, TXT and DATA Allowances that encompasses!
See instructions below on how to do that in future with your Airtime Credit Topup Balance.
Buying a goodybag from credit by phone
Dial 43430 from your handset.
At the main menu Press 2 to buy a goodybag and choose to use your Airtime Credit for the Goodybag purchase.
As we in the forums are just Members like yourself, we cannot action any direct Account Related issues, therefore, please make contact with a giffgaff Agent who should be able to help you with this isssue. You can do so via this link : https://support2.giffgaff.com/app/ask/Top-up-balances-and-goodybags/Cancelling-a-goodybag-or-top-up-purchase/form/ and advise them of what has happened in detail and they may refund you as a goodwill gesture!