Preheat the oven to 220C/425F/Gas 7 (200C fan).
Sprinkle the sugar over the base of the cake tin in an even layer.
Arrange the plums on top of the sugar, cut-side down.
Roll out the marzipan thinly to a round slightly smaller than the tin and place on top of the plums.
Roll out the pastry just a little bit bigger so that it is the width of the cake tin. Cut around the tin to make a circle, lay the pastry over the plums and tuck the edges of the pastry down around the fruit. Make a small cross in the top of the pastry to let the steam out.
Bake for 25-30 minutes, or until the pastry is crisp and golden-brown and the plums are tender.
Loosen the edges of the tart, then turn out onto a plate and serve with ice cream, cream or crème fraîche.