Grease a 20cm square baking tin. Measure a double layer of foil long enough to line the base and 2 sides of the tin with 20cm excess. Fold a 10cm pleat in the centre (step 1). Push the foil into the tin, making sure the pleat is in the centre and the corners are nice and sharp (step 2). Line the 2 halves of the tin by putting a long strip of baking parchment in lengthways over the foil (step 3). Criss-cross another strip widthways to completely cover the foil (step 4). Repeat to line the other compartment. You now have two compartments for the different colours.
Heat oven to 180C/160C fan/gas 4. Put the butter and sugar into a bowl and, using an electric whisk, beat until light and fluffy. Beat in the eggs, one at a time, then beat in the flour, baking powder and almond extract. The batter needs to be dropping consistency (it should fall off a wooden spoon). If it is a little thick, add a drop of milk and continue to beat.
Halve the batter by weighing and dividing equally into separate bowls. Fold the red colouring through one and the yellow colouring through the other.
Tip the batters into each side of the prepared tin and bake for about 25 mins, until the sponges have risen and a skewer inserted into each sponge comes out clean. Leave them to cool in the tin, then remove to a rack to cool completely.
To assemble, neatly trim each sponge, then sit one on top of the other and trim again so they are both the same size. Cut each sponge in half lengthways so you have 4 long rectangles. Warm the jam in a small pan, then brush along a long side of each sponge. Stick the jam sides together to create the chequerboard effect, then brush the sides and top with more jam.
Dust a work surface with icing sugar. Roll out one of the blocks of marzipan until it is
20cm long, then roll it so it is wide enough to wrap around the sponge. Brush the loaf
all over with more apricot jam, then tightly wrap the marzipan around the sponge,
trimming it where the edges meet. Smooth the marzipan over the sponge. Sit the loaf
on its seam.
To make the Apostles, roll the remaining marzipan into 11 small balls (you might not need all the marzipan for this, but any excess can be frozen) and sit on the loaf at even
intervals. To give the cake a contrast of colour and a classic simnel cake finish,
lightly blowtorch the balls on the top of cake. The cake will now keep for up to 3 days
in an airtight container.