Cut the pastry in two. Wrap one half and set aside. On a surface lightly dusted with flour, roll out the other half to a square large enough to line a 22cm loose-bottomed fluted tart tin (we used a square tin, but a round one would also work). Press the pastry into the tin, leaving any excess hanging over the edge.
Scatter the marzipan over the pie base. Add the mincemeat and top with the cranberries.
Roll out the remaining pastry to a square (or circle) the thickness of a £1 coin. Use a pizza cutter to slice the pastry into 1.5cm-wide strips. Weave the pastry strips over the pie to create a lattice, pressing down at the edges to seal with the pastry base. Trim off any excess pastry. Chill for 20 mins and heat oven to 180C/160C fan/gas 4.
Bake for 45 mins until golden. Cool for 15 mins in the tin, then transfer to a plate or board and dust with the icing sugar.