Rinse and completely dry the cherries on kitchen towel. Sift the flour and baking powder into a bowl and set aside. Roll out the marzipan between 2 sheets of clingfilm into a very thin rectangle or circle, slightly smaller than the tin, and set aside.
Cream the butter, sugar and zest together in an electric mixer until very pale, light and fluffy (this will take about 5 minutes).
Beat in the eggs a little at a time, adding 1 tbsp flour during the process to stop the mixture from curdling. Fold in the almonds, the sifted flour mixture and the lemon juice until well blended, then spoon half the cake mixture into the tin. Place half the cherries over the mixture in a shape echoing that of the tin, towards the middle. This will support the marzipan. Lay on the marzipan, then add the remaining cherries, again in a shape echoing the tin, but this time towards the outer edge. Spoon on the remaining mixture. Sprinkle over the demerara and
Cool the cake in the tin for 15-20 minutes, then turn out and remove the paper. Leave to cool completely. Don’t worry if it sinks a bit in the middle; it’s due to the weight of the marzipan and cherries. If decorating, simply invert it so the flat base forms the top.