Pre-heat the oven to 200°C. Place the plum halves, cinnamon stick, ground allspice and sugar in a large saucepan. Add 100ml of water, cover with a lid, and bring to a simmer over a low heat. Remove from the heat once simmering and mash lightly to break up the fruit, discarding the cinnamon stick at the same time.
Brush 4 of the sheets of filo pastry with some of the melted butter. Layer, overlapping if necessary, inside a 7 inch springform tart tin as evenly as possible, shaping into the sides and base carefully.
Spoon the plum compote onto the pastry, then brush the remaining 2 sheets of pastry with more of the melted butter. Layer on top of the plum compote so that it covers that tart and acts as a lid. Brush the top of the pastry with the rest of the melted butter, then sprinkle the caster sugar evenly on top.
Bake for 20-25 minutes until the pastry is golden brown and cooked. Remove and allow to sit for 5 minutes before turning out onto a serving plate.