The general answer would be NO. But depending on the circumstances that you put the credit on, You could always try asking an agent from the following link
Explain the situation to the agent and see how you go, Whats the worst that could happen ? they might say no , but you will not know unless you ask.
I had a problem a couple of weeks ago with my queued goodybag not activating on the date it was supposed to and with no airtime balance on my account, my phone was next to useless . I needed to make some phonecalls so I had to pop out and purchase another £10 top up voucher . Annoyingly, by topping up my phone it seemed to activate my goodybag.
I asked the community if I would likely be able to get it refunded and everyone here said "NO" but I asked an agent anyway, To my surprise, they agreed to refund me the £10 via the debit card linked to my account.
( A BIG, BIG Thank you to Giffgaff for refunding me, really appreciated !!!)
You don't get anything if you don't ask!