VirginMedia took too much money from my account but will only give me account credit NOT put the money back in my bank account!! What can i DO?

As title really, they took £100 too much from my account, they appolgise but can only put it back in credit on my account, not back into my bank account..

Is there anything i can do?

My monthly bill is only 18.75 a month, seems like they are taking the pee thinking they can get over 5 months interest free credit (paying off the 100 @ 18.75 a month!)

Told them thats not good enough, but not sure what else i can do??