Roll out the pastry thinly on a lightly floured surface and use to line the base and sides of a 28 x 18 cm (11 x 7 inch) shallow baking tin. Chill for 30 minutes.
Line the pastry case with nonstick baking paper and baking beans. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Remove the paper and beans and cook for a further 5 minutes. Reduce the oven temperature to 180°C (350°F), Gas Mark 4.
Spread the cherry conserve over the base of the pastry case. Put the butter, sugar, eggs, almond extract and ground almonds in bowl, sift in the flour and baking powder and beat with a hand-held electric whisk until smooth and creamy. Stir in the cherries.
Spoon the filling out into the pastry case, spreading it gently so you don't dislodge the conserve. Level with the back of a spoon and scatter with flaked almonds. Bake for 40 minutes until risen and just firm to the touch. Leave to cool in the tin.
Make the glaze by beating together the icing sugar and lemon juice. Remove the cake from the tin and drizzle with the glaze. Serve cut into small slices.