Thinly slice the butter and divide it among 4 small heatproof ramekin dishes. Sprinkle with the sugar. If the redcurrants are still attached to their stalks, reserve 4 clusters for decoration and remove the rest from the stalks (the easiest way to do this is to run the currants between the prongs of a fork). Scatter several redcurrants into each dish, then add the pears.
Unroll the pastry and cut out rounds using a 10 cm (4 inch) biscuit cutter. Lay the pastry rounds over the pears, tucking the edges down inside the dishes.
Place the dishes on a baking sheet and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 15–18 minutes or until the pastry is well risen and pale golden. Using oven gloves, remove the baking sheet from the oven and put it on a heatproof surface. Leave to cool slightly.
Loosen the edges of the pastry with a knife. Hold a ramekin dish with oven gloves and invert a small serving plate on top. Carefully flip over the dish and plate so that the plate is the right way up. Lift off the dish to reveal the tart with its fruity topping. Repeat with the remaining tarts. Serve decorated with the remaining redcurrants.