BA COMAIR LIES SO I WOULD BUY ANOTHER TICKET

Bought return ticket JHB to Cape Town on 16 April. Received sms from bank with confirmation of payment as well as confirmation letter from webjet. Departed on Thursday 30th Kulula Airlines NO PROBLEM. Sunday morning 3 May when I checked in I was told by BA that I wasn't on system. Referred to BA sales ticket counter and I was then told my ticket was cancelled because I didn't pay it. I showed them the sms, the confirmation letter which I btw used on Thursday to fly OUT of JHB. I was then charged R3000 for a ticket after which my husband asked to see the manager. More than an hour later I had a new ticket which I had to pay R2079.72 for. Today I phone webjet just to hear that I have an open ticket. Which means I never used my ticket. Told them what happened and that I want a refund. Still waiting for response. Apparently BA overbook their flights and then this happens!!! I just want my R2079.72 back from BA as THEY made the mistake. Even after showing the lady ALL THE PROOF WE PAID AND HAD A TICKET CONFIRMATION SHE STILL TOLD ME I HAD TO BUY A NEW TICKET!!!!! I will NEVER EVER FLY BRITISH AIRWAYS AGAIN!!!!