Halve the pastry, roll one piece out to approx 16 × 20cm and lift it onto a baking sheet. Roll the marzipan out and lay it in the centre of the pastry, leaving a gap around the edge. Spoon damsons on top and sprinkle over the ground almonds. Brush around the edge of the pastry with egg, then roll out the second piece of pastry and put on top. Seal the edges, press them down, then trim the excess. Use a sharp knife to slash the top of the pastry in 2 or 3 places. Brush the top with egg and chill the slice for 20 minutes.