Roll out the pastry on a lightly floured surface to a 35 x 20 cm (14 x 8 inch) rectangle. Place on a greased baking sheet. Using a sharp knife, score a 1.5 cm (¾ inch) border around the pastry edge. Roll out the marzipan on a surface lightly dusted with icing sugar to a 30 x 15 cm (12 x 6 inch) rectangle, then place inside the border. Arrange the plums over the marzipan. Brush the pastry border with beaten egg.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 25 minutes until the pastry is risen and golden and the plums are tender.
Meanwhile, heat the glaze ingredients in a small saucepan, then brush over the plums. Serve warm or cold.