Grease a 20cm loose bottomed cake tin with butter and line the bottom with baking paper. Make a crumble by rubbing together the flour, butter, icing sugar, lemon zest and vanilla together. Spoon just over half into the base of the cake tin and take up the sides by an extra 1cm. Spoon the cold custard on top followed by half of the frozen berries. Mix the remaining frozen berries into the remaining crumble mixture and spoon on top. Finish off with the flaked almonds on top. Bake in the oven at 180`c for approximately 35-40 minutes. Meanwhile, whip together the double cream, vanilla and icing sugar to soft peaks. When cooked, allow the crumble to sit for 5 minutes before cutting into wedges and serving with a dollop of the vanilla cream on top.