Mango Airlines poor service

I have been a loyal customer for some time and travel between Durban and Cape Town regularly. I flew to Cape town on Friday 16/08 with the return flight booked for Saturday 17 / 08 13H15 flight JE348. not only was this flight delayed for 3 hrs I had paid R 55 for the seat booking and was allocated the seat " 3 D " . When I came to book in I was told that my seat had been given to another passenger and it could not be changed. They also said that although the seats in row 1 and 2 where empty they could not give me one of these as they had been blocked due to load " weight " balancing of the plane. All the above was a lie as the seat where all occupied and my seat was given to a passenger who booked in after me. Flight staff not interested in helping solve their problem. Very poor and disappointing service.