Go to your My Giffgaff page and look for the agent links on the right hand side. Explain your situation there.
You will have to contact an agent mate regarding refunds.
You can either get a refund through the agents, or you can just leave the extra credit on your account and use it up. If you're buying a goodybag each month it's amazing how the credit goes down 😀
http://support.giffgaff.com/app/ask << Go to this link mate! 🙂