Charterguy - If an airline gives passengers 14 days of more notice then they are not required to offer compensation.
The general rule is you get a refund but many airlines will offer an alternative if they operate the same route elsewhere. In this case I do not believe GSM fly to Cape Town other than Manchester.
As far as I am aware, if there is not 14 days notice compensation is required unless the airline can rebook you on another carrier and your arrival is within 2 hours of the flight which was previously booked. I hope this helps.