Pre-heat the oven to 170°C/ 150°C fan/gas mark 3.
Place the cranberries in a medium sized saucepan with the water and cook on a low heat for about 15 minutes, stirring occasionally until the cranberries have broken down.
Blend the cranberries until they are smooth then set aside in the fridge to cool completely.
Melt the butter in a small saucepan and leave to cool for a few minutes just so the butter is warm rather than hot which helps your biscuit base from becoming too greasy.
Whizz up the biscuits in a food processor then with the mixer still on pour in the melted butter until it combines with the biscuit to become a thick sandy texture.
Press the biscuit crust into a 23cm pie dish using the back of a spoon and push the base up the sides of the dish.
Bake the crust for 18 minutes then remove from the oven. Gently re-press and shape the biscuit crust which might have puffed up a bit and then leave to cool in the fridge to set whilst you make the filling.
Turn the oven down to 160°C/140°C fan/gas mark 1.
Mix the cranberries, lemon juice, egg yolks and condensed milk in a large mixing bowl.
Pour the cranberry filling into pie shell, smoothing out the top.
Bake the pie for 25 minutes, the cranberry filling should still be a little wobbly.
Leave to cool and set in the fridge overnight.
Whip the cream until thick and it can hold its shape then pipe over the top of the pie.