Put raisins in a heatproof bowl and cover with boiling water.
Roll out the pastry on a lightly floured surface so it's slightly thinner than straight from the pack. Then cut out 24 rounds with a 7.5cm fluted cutter, re-rolling the trimmings. Use the rounds to line two deep 12-hole tart tins (not muffin tins). If you only have a regular-sized, 12-hole tart tin you will be able to make a few more slightly shallower tarts.
Beat the eggs, butter, sugar, syrup, vanilla and salt in a large bowl until smooth.
Drain the raisins and evenly distribute them in the bottom of the tart shells.
Spoon the filling over the raisins in the unbaked tart shells so they are 2/3 filled.
Bake until set and lightly golden, 15 to 18 minutes. Leave in the tin to cool for a few minutes before lifting out on to a wire rack. Serve warm or cold.