1. Place the flour, salt and margarine into the processor bowl and process together for 10 seconds (breadcrumb stage).
2. With the machine running slowly add the water down through the feed chute, until the mixture starts to form little tiny balls. At this stage stop adding the water and allow the food processor to continue mixing until a dough ball is formed.
3. Roll out the pastry and use to line a 20cm (8 inch) flan dish or pie plate, reserving trimmings.
4. Warm the syrup, treacle and lemon juice in a saucepan.
5. Break the bread into pieces, place in the processor bowl and chop finely into breadcrumbs.
6. Add the warm syrup to the breadcrumbs and mix together well. Pour into the pastry case. Roll out the pastry trimmings, cut into long strips and arrange in a lattice design on the tart. Brush pastry with milk.
7. Bake in the centre of the oven at 200°C, 400°F, Gas No. 6 for 20-30 minutes.