Grease a 1lb loaf tin with oil and add half the strawberries. Put the orange jelly in a measuring jug and add boiling water up to the 200ml mark. Stir until dissolved, then top up with cold water to the 250ml mark. Allow to cool, then pour half the jelly over the strawberries. Leave to set in the freezer for 10 minutes.
Remove the jelly from the freezer and top with the blueberries. Make the strawberry jelly as above and pour half onto the blueberries. Return to the freezer for 10 minutes.
Place the remaining strawberries on top. Pour in the remaining orange jelly and leave to set in the fridge for 2-3 hours or overnight.
When ready to serve, remove from the fridge and quickly dip the loaf tin in warm water. Turn out the jelly onto a board. Slice and serve with the orange segments and mint leaves.
Cook's tip: pour the leftover strawberry jelly into ramekins and keep in the fridge for a quick dessert the next day.