Heat the oven to 180C/fan 160C/gas 4. Roll out the pastry to 20p coin thickness then use to line a long rectangular tart tin approx 35cm x 12cm, or a round tin approx 23cm. Line with baking paper then fill with baking beans. Chill in the fridge for 20 minutes. Bake for 15 minutes then take out the paper and beans and cook for another 5 minutes until sandy coloured and cooked. Take out of the oven.
Beat the butter and sugar together then gradually beat in the eggs. Mix in the ground almonds and almond extract. Sift the self-raising flour and baking powder and fold in.
Spread a thin layer of jam over the base of the pastry. Spoon over the almond mix then level out. Bake on a lower shelf for 30-40 minutes until set.
Mix the icing sugar with just enough water until it coats the back of a spoon (about 1 tbsp). Spread a layer of icing over the top of the tart, going right up to the pastry edge, then leave to set. Decorate with cherries.