1 For the pastry: sift the flour into a mixing bowl and stir in the sugar. Rub in the butter until the mixture resembles breadcrumbs. Gradually add the water, a tablespoon at a time, mixing continuously until the mixture just comes together as a dough. (You may not need to use all of the water. ) Roll the dough into a ball, then wrap in cling film and chill for 1 hour.
2 Heat the oven to 190°C (170° fan) 375°F gas 5.
3 Roll out the dough on a floured surface and line the base and sides of the tin.
4 Mix together the damsons and sugar and put into the pastry case. Bake for 15minutes
5 For the topping: mix all the ingredients together with a fork until combined. Place spoonfuls of the topping on the filling and bake for a further 15 minutes until the damsons are soft and the pastry is golden. Cool in the tin.