British Airways Comair
Refund processed money still not recevied 4mnths

To whom it may concern,
I am utterly disgusted by BA. A refund that they claim they have paid still doesn't appear in my bank account after 4 months.
They made a mistake and refunded me only for one ticket, then acknowledge the mistake and instead of refunding me for both tickets reversed the first refund but still sending me proof that they have refunded both tickets.
BA says check with bank and the bank has done everything possible to proof that the refund was not received by them. All BA is doing is ignoring our calls and no one takes ownership of the issue and says they have done their bit I need to check with my bank.
What must I do more to proof to BA that the money that they apparently refunded to us hasn't been received by us and that the bank has checked and can't find any track of such a refund?
The Comair service desk is unprofessional and not customer friendly. I will go out of my way not to use any British Airways or Comair service ever again and influence any relation of mine to do the same.
We are being bullied by an uncooperative corporate with its processes, procedures and no personal accountability within the organisation.