Photograph: Dan Jones
Preheat the oven to 180°C, fan 160°C, gas 4. Pour boiling water into a roasting tin (to 2cm deep) and place it on the bottom shelf or the base of the oven.
Grease and line a deep 23cm loose bottomed cake tin. Trim around the edge of the flan sponge so that it fits snugly in the base of the tin, then spoon over the Grand Marnier.
In a large bowl, whisk together the cream cheese, sugar and vanilla seeds until combined. Beat in the eggs and cornflour.
Once combined, add the whipping cream and whisk until thick.
Pour into the prepared tin. Bake on the middle shelf for 50 minutes until set. Leave to cool completely in the tin. If you're not freezing the cheesecake, chill in the fridge for at least 5 hours before serving.
Prepare the topping. Place the cranberries, mixed berries, sugar and cinnamon in a large pan. Slowly bring to the boil to allow the sugar to dissolve, then reduce the heat and simmer for 5 minutes, stirring occasionally. Remove from the heat, transfer to a freezable lidded container and allow to cool.
To freeze, wrap the cooled cheesecake (in the tin) with a double layer of clingfilm. Freeze the cheesecake and the mixed berry topping separately.
The night before you want to serve it, transfer the cheesecake and topping from the freezer to the fridge to defrost. Put the cheesecake on a plate or cake stand and spoon the berries over the top.
Get ahead: the cooled cheesecake can be frozen (in the tin), wrapped with a double layer of clingfilm. Top with the berries just before serving.
Kitchen secret: we've cheated for speed and used a ready-made flan sponge as the base for this cheesecake.