Appalling customer service

I'm absolutely disgusted by the service received by Mango Airline. I was booked on the 11:45 flight from Cape Town to Durban yesterday. My daughter and I arrived and joined the queue at least 50 minutes before the flight to allow sufficient time for checking in. The queue was barely moving and after some time I approached a staff member to inform her that we are on the Durban flight and surely we should be checked in already. They then moved us to a shorter queue but by the time anyone assisted us we were told that the flight had closed for check in and there is nothing they can do for us, this was 35 minutes before the flight departure. I explained that I had already been waiting in the queue for at least 15 minutes before that and was there well within the time stipulated for check in. They imp**** that I was lying because they said the flight controller would have approached each traveller individually to ask what flight they were on. He certainly didn't and there were no announcements made either. I then asked to speak to a manager as I wanted her to check the camera footage to prove that we were there well within the required time. She also imp**** that we were lying saying it was their word over ours, which again is appalling customer service. She said she could check the footage but it would take a very long time. I asked to see the footage myself because at this stage I knew exactly where this was going. She told me this is not possible ...how convenient since this is my only proof! They then offered to put me on standby for the next flight at 3. This was fully booked and the next one would be at 6pm but after being informed that there were others in the queue in front of me, I realised that the chance of us getting another flight was almost impossible. We had already spent the most of the day at the airport and I had to also cancel my daughter's birthday party that was supposed to be on Friday evening. Not only is it extremely inconvenient, since Mango were fully booked for the next few days, I've had to book another airline at a premium rate because of peak season. Mango has cost me over R6000 and given me the worst customer service ever received. I would like someone to check that footage and prove that we were there and I expect a refund for my flight since they clearly sold my seats to other passengers. But then again, it's their word over mine and I suppose that's how they will continue to deal with these situations where they are clearly at fault. I now realise why so many people refuse to fly Mango. I certainly won't be in future!