Heat half the sugar together with the vinegar and half a litre of water. Prick the kumquats with a fork and simmer for 15 minutes in the vinegar mixture. Make some caramel with the remaining sugar and mix in the butter. Add the pineapple and fry for 4-5 minutes, stirring occasionally. Chop the mango flesh and add to the compote. Serve with the caramelised pineapple.