Heat oven to 160C/fan 140C/gas 3 and butter and line a 20cm round, loose-bottomed deep cake tin. Beat the butter and sugar together with electric beaters or a tabletop mixer until light and fluffy. Add the eggs, 250g flour, baking powder, orange zest and juice and beat well till thick, creamy and evenly mixed.
Toss cherries in the extra 2 tbsp flour and fold into the batter along with grated marzipan and almond essence (if using). Spoon into the prepared tin, level and bake for 1 hr 30 mins, or until well risen and golden. A skewer will come out clean when ready. Leave to cool in tin for 10 mins, then turn onto a cooling rack to cool completely.
Mix the icing sugar with just over 1 tbsp water to make a loose but not-too-runny icing. Scatter the almonds over the top
of the cake and position the 12 glace cherries around the edge, using a dot of the icing to fix. Drizzle the icing over the cake and finish with a scattering of orange zest.