Lay out the sponges on the bottom of a large glass bowl (if you're using a Swiss roll, cut it into slices first). Hull and slice the strawberries, then scatter over the sponges and drizzle with the Cointreau.
Tear up the strawberry jelly squares, put them in a jug and add 300ml boiling water. Stir until dissolved, then add 300ml cold water. Pour over the sponge, place in the fridge and allow to set.
Pour a little milk into a bowl with 2 teaspoons of sugar and the blancmange and mix until dissolved. Bring 600ml milk just to the boil then add to the blancmange mix. Stir and return to the pan, whisking as you bring it back to the boil. Pour the blancmange into the bowl and allow to cool, whisking occasionally, then pour over the set strawberry jelly.
Allow the blancmange layer to set in the fridge, then drain the mandarins and scatter over.
Make the orange jelly the same way as the strawberry and allow to cool before pouring over the mandarins. Set in the fridge.
Once set, make the custard using the remaining milk and sugar and the custard powder. Allow it to cool and pour it over the orange jelly. (If you want to add extra layers of blancmange and custard, go for it!)
Whip the double cream until you get soft peaks, then spread it over the top layer of custard. Grate chocolate and orange zest over the trifle before serving. What a finish!