One of my Dad's specialities, the tricky part is getting the mixture to set.
Crush the digestives and add them together with the 1.5oz of castor sugar to the melted margarine, mix together. Then put this mixture into the bottom of your tin, making it as level as possible.
Cream together the egg yolks, sour cream and cream cheese - put this mixture to one side. Whisk the egg whites till they are 'soft peaks' and then add the 5oz castor sugar to them. Next add the juices and rind of the lemon and then the gelatine + water mixture. Fold the cream cheese mixture into the egg whites mixture, and place it all on top of the biscuit base. Then put it in the fridge to cool.