Change of flight times

I made a booking in January for the 31st of March to 4th of April 2016. Someone from Mango called on the 29th of March to advised that the flight from Cape Town back to Johannesburg will move from 12:45 to 13:30 and I asked them to send it in writing. I then received a blank email! On the 29th of March I received an email stating that the flight time is 16:45. That is such an inconvenience for all 3 of us and who gave permission for those changes anyway? We not even told why, I need an explanation.