Preheat the oven to 200°C (Fan 180°C) / 400°F / Gas Mark 6.

In a large mixing bowl, combine the flour and dairy-free margarine. Using your fingertips, rub the ingredients together until the mixture resembles fine breadcrumbs. Gradually add the cold water, a small amount at a time, mixing with a table knife after each addition. Cut through the mixture rather than kneading it to avoid toughening the pastry. Once the dough forms a soft, smooth ball, place it in the refrigerator to rest for 10 minutes.

After resting, roll the pastry out on a lightly floured surface to a thickness of approximately ½ cm. Using a 9 cm round pastry cutter (or similar-sized glass), cut out 12 pastry circles. Gather the trimmings, re-roll as needed, and continue cutting until 12 circles have been formed.

Place each pastry circle into a muffin tin, pressing gently to form a shallow tart shell. Divide the jam evenly among the 12 shells, taking care not to overfill, as the jam will expand during baking.

Bake for 15 minutes, or until the pastry is lightly golden and the jam has melted. Remove the tray from the oven and allow the tarts to cool completely before serving, as the jam will be extremely hot immediately after baking.