Preheat oven to 170ºC / 325ºF / Gas Mark 3. One of the biggest challenges is to line the tin properly. Grease and line the square tin with baking parchment. Divide the tin in half with a strip of foil folded into a double thickness. This will allow you to bake the two halves separately.
In a large mixing bowl, beat the margarine and sugar to a soft cream. Gradually add the beaten eggs and flour to the mixture, adding in a little of each alternately. Beat to a smooth batter.
Transfer half of the batter into a second bowl, and colour this pink with a few drops of red food colouring.
Pour the separate batches of batter into each side of the baking tin and bake in preheated oven for 40-50 minutes, or until a skewer inserted into the cake comes out clean.
Turn onto a wire rack and leave until quite cold. Cut each sponge lengthways into two perfectly rectangular strips, trimming if necessary to ensure equal sides.
Taking one pink and one white strip, brush one length of each with jam loosened with a little hot water. Press together to form the base.
Brush over with jam, top with two more strips (the plain strip on top of the pink and vice versa) and press together to bind.
Place a dish, or a tin with a small weight in it, on top of the cake and leave for an hour.
Meanwhile, make the marzipan if not using shop-bought paste. Measure the outer surface of the cake. Roll the almond paste out on a sugared surface, aiming for a piece large enough to cover the cake completely.
Brush the top and sides of the cake with jam and place topdown on the paste.
Brush the final side of the cake with jam and wrap the paste around, pressing to secure.
Pinch to seal the join and then pinch the edges to define the shape. Cut into slices to serve.