Butter a 20cm round springform tin thickly. Turn tin on its side, place a few digestive biscuit crumbs in tin and shake so crumbs stick to sides. Keep turning and add crumbs as needed until sides are well coated. Set tin upright and coat bottom of tin. Put tin into freezer for 30 minutes or refrigerator for 2 hours.
Preheat oven to 150 C / Gas 2.
With electric beater, mix cream cheese, sugar, egg yolks, flour, vanilla and salt. Beat thoroughly and then turn mixer to slowest speed and slowly pour in the cream.
Beat the egg whites until stiff but not dry. Add to the above mixture and fold the whites in by hand, lightly but thoroughly. Pour mixture into the crumb-lined tin.
Set tin into shallow baking tin containing 2.5cm hot water and bake at 150 C / Gas 2 for 1 1/2 hours. Refrigerate for at least 6 hours before serving.