Step 1Make the blackberry compote. Put 200g of the blackberries into a saucepan with the icing sugar. Turn on the heat and gently mash the blackberries to release the juice, then bring up to the boil. Turn down to a simmer and cook until fruit is breaking down and juices thicken, about 4-5min. Take off the heat and set aside to cool, before gently stirring in the remaining blackberries and the gin.
Step 2To assemble the trifle, slice the swiss roll into 1.5cm (2/3in) slices and arrange around the sides of a 2litre (31/2pint) glass bowl. Break up remaining slices to roughly fill the middle. Drizzle over 3tbsp of the sloe gin and leave to soak in.
Step 3Beat the blackberry jam with the remaining sloe gin in a bowl and spoon half the jam in random blobs over the cake layer. Spoon the most of cooled blackberry compote over the top, then dot the rest of the jam over randomly. Pour the custard over, then chill, uncovered, for at least 30min to firm up.
Step 4When ready to serve, lightly whip the cream with the remaining icing sugar and most of the orange zest until it is just holding it’s shape. Spoon on to the custard layer. Decorate the top with extra blackberries, remaining blackberry compote and the extra orange zest.
GET AHEAD
Make trifle up to step 4 a day ahead, but don’t decorate. The unassembled trifle will keep for up to 3 days in the fridge.