Place the gelatine leaves in a bowl with cold water for 10 minutes. Meanwhile, grease a 20cm square, 2cm deep baking tin with sunflower oil.
Heat the conserve in a saucepan on a low heat until simmering. Remove from the heat and add the gelatine mixture. Stir briskly to melt the gelatine, then pour into the tin.
Leave to stand for two to three hours, or overnight. Remove from the tin with a spatula, dip the blade of a sharp knife into hot water, and slice the jelly into 3 x 3cm cubes.
Roll the cubes in sugar to finish, and serve. The crystallised fruit can be kept for up to 3-4 days in an airtight container.