Double debits and no refund!

I had a debit order with On Air Tv that never went off so I paid the amount in manually. After paying the account, their customer care constantly phoned me to tell me to pay my account. I told them it had been paid and that I had even sent them proof of payment. They then went an double debited my account even though I had made payment. I have been trying for over a month to get the money back, emails hardly ever get rep**** to and even though I was told the refund would be processed, still nothing had come through. When you try phone them, customer care does not answer so it’s impossible to even try liaise with them telephonically! The one email I received said it would be processed last week latest, still nothing! On Air, your service is beyond disgusting! I will be changing my banking details so you cannot debit my account until you refund me!