Overcharged - spoke to support for "fix" rinse and repeat for 8 months

Hello.

Yet again I've been overcharged by virgin.

I've typed this so many times to so many service reps, I'm getting tired of it.

Long story short: my contract renewed with a charge of £21.00

Virgin decided it should be £44 so charged me for it.

When I realised, they said they will give me rolling credit to bring it back down.

Still got charged.

Finally said £0 charge until my overpayment goes to zero, then rolling credit.

I moved, before moving made sure the credit would still apply. TWO service reps said it will.

First bill after moving: charged again £44. Called them again. Again the rep said "so sorry I'll resolve it now don't worry rolling credits"

Month two after moving: service rep says "oh no actually it's a new contract and all credit gets wiped when you move" and just ended the chat. 

I've overpaid £60 now. Virgin seems unable to help me. I've spent literally tens of hours after work speaking to reps every month trying to get this fixed. Each time they acknowledge it was a mistake. Yet it still doesn't get fixed.

Is my next option to go to the ombudsman or seek legal advice?