Sift the flour and salt into a bowl. Cut the margarine into small pieces and rub in until the mixture resembles fine breadcrumbs.

Sprinkle over the water and mix to form a dough. Knead very lightly, roll out three-quarters of the pastry on a lightly floured surface and use to line a 23 cm (9 in) ovenproof pie plate.

Leave to chill in the refrigerator while preparing the filling.

Gently heat the syrup and mix in the lemon rind and breadcrumbs. Pour into the pastry shell. Roll out the remaining pastry and cut into thin strips to form a lattice pattern on top of the tart.

Bake in a moderately hot oven for about 30 minutes and serve with a custard sauce.