First make the crumble. Preheat the oven to 180°C/350°F/Gas Mark 4 and grease a baking sheet. In a bowl, mix together the flour, sugar and margarine using a spoon or your hands until you have a lumpy crumble. Spread it out evenly on the baking sheet and bake in in the oven for 8-10 minutes or until the crumble is nicely browned. Remove from the oven and cook in the baking tray on a wire rack. Grease two round 20cm, 23cm or 25cm (8”, 9” or 10") cake tins. In a large bowl, mix together the flour, sugar and baking powder for the sponge. Add in the soya milk, the oil, the rhubarb and the vanilla, stirring by hand until just combined. It is important not to mix the batter too much or your cake won’t rise properly. Tap the bowl on to the work surface to stop the raising agents working too quickly – you will see the bubbles pop. Spoon half the cake mixture into each of the cake tins and tap the tins on the work surface to pop the bubbles again. Bake for about 18-20 minutes or until a toothpick inserted into the middle of the cake comes out clean. Cool for about 10-15 minutes in the tins then transfer the cakes to a wire rack and cool completely. Make the custard buttercream icing. Using an electric or a hand-held mixer, whip together the margarine, vegetable fat and the vanilla until creamy (about 30 seconds). Add half of the icing sugar, the custard powder, 40 ml (2 tbsp + 2tsp) milk, and the vanilla and continue mixing, slowly at first and then bringing up to speed, until combined. Add the rest of the icing sugar and mix for about a minute until it is a smooth consistency. If the icing is too firm, mix in a bit more milk. If it is too soft, add a bit more icing sugar. Take the bottom layer of your cake and spread liberally with some of the Custard Buttercream icing. Place the second layer of cake on top. Decorate the sides of the cake with more of the buttercream and coat the iced sides in the prepared crumble, saving some for the top, if you like. Smear the remaining buttercream on top of the cake and work the rhubarb into the remaining icing as you spread it out – this will add flavour and a lovely colour. Decorate with traditional rhubarb and custard sweets or sprinkle with some more crumble.