Grease a 8in (20cm) loose bottom tin and line the base with grease-proof paper.
Melt the margarine with half of the plain chocolate in a bowl over a pan of simmering water.
Melt 7 oz (200g) of the white chocolate in the same way. Allow to cool slightly.
Whisk together 2 eggs and the sugar and beat in the plain chocolate mixture until smooth. Fold in the flour and pour into the prepared tin.
Whisk together the cream cheese and remaining egg, then beat in the melted white chocolate.
Spoon into the tin on top of the brownie mixture. Using a skewer, carefully swirl the two mixtures to create a marbled effect.
Bake in the oven for 50–55 minutes until just set. Run a knife around the sides of the cake, allow to cool slightly, then remove from the tin. When completely cold remove the base and the grease-proof paper.
Melt the remaining white and plain chocolate separately. Spread a little of each over the top of the cheesecake. Decorate with the marshmallows and maltesers. Drizzle the remaining white and plain chocolate over the top and chill until required.