Butter a 23cm springform cake tin.
For the base
Mix together the biscuit crumbs and melted butter.
Press firmly into the base of the tin.
Chill while you make the filling.
Heat the oven to 140°C (120° fan) gas 1.
For the filling
Whisk together all the ingredients. Pour onto the biscuit base.
Cover the tin with foil and pierce a couple of holes in it.
Bake for about 50 minutes until firm around the outside but with a wobble in the centre.
Cool completely in the turned off oven, with the door slightly open.