Grease a 9" (23cm) spring-form tin very well. Preheat oven to 180 C / Gas 4. Combine crushed biscuit and melted butter; if it's too wet add some more biscuits. Press evenly into spring-form and 2cm up the sides. Chill.
The Filling: Place cream cheese, sugar and flour in a bowl, beat on medium speed until smooth (about 2 minutes). Add eggs one at a time beating well (about 30 seconds). Add cream, zest and vanilla beat until just mixed in. DO NOT OVERBEAT!
Remove tin from fridge and place on an oven tray (to catch leaks) add filling on top of biscuit. Place in centre of oven, bake at 180 C for 15 mins THEN LOWER TEMP to 120 C / Gas mark 1/2 and bake for a further 1 1/2 hours until firm and only centre of cake looks wobbly and a little wet. Remove from oven.
Meanwhile combine creme fraiche, sugar and vanilla. Spread evenly over cheesecake and return to oven for 15 mins more. Remove from oven and very carefully run a knife around the edge to loosen the cake from the tin.
Leave to cool before removing from the tin. Once cool run the knife around the edge again and very carefully loosen the spring-form so as not to crack the cake. This may require some skill especially where the tin overlaps.
Cover with clingfilm and chill overnight. Decorate with fruit of your choice to serve.
Place chilled cheesecake on a tray and freeze uncovered until firm. Remove from freezer and wrap in heavy duty foil, place in a freezerbag and seal. Return to freezer. To thaw - uncover and place in fridge overnight
CRACKING occurs when you over beat the batter or over bake. Make sure the sides are well greased. I usually have a problem with this when removing the tin, but use a flat knife at the tin joint whilst slowly removing the tin - you may need a third hand for this!