To make the shortcrust base: Combine 125g of the flour, 4 tablespoons of the sugar and 1 teaspoon grated lemon zest. Rub in the butter until the mixture resembles coarse crumbs. Stir in the beaten egg yolk and 1/4 teaspoon vanilla. Pat 1/3 of the mixture into the bottom of one 23cm springform cake tin with the sides removed. Bake at 200 C / Gas 6 for 6 minutes. Let cool then butter sides of tin and attach to bottom. Pat the remaining mixture evenly on the sides to a height of 5cm.
To make the filling: Preheat oven to 260 C / Gas 9. Stir cream cheese to soften, then beat until fluffy add 1/8 teaspoon vanilla and 1/2 teaspoon grated lemon zest.
Mix 1 1/2 tablespoons flour, 1/8 teaspoon salt and the remaining sugar together. Gradually blend this mixture into the cream cheese mixture. Add the eggs and the egg yolk one at a time, beating well after each one. Gently stir in the double cream. Pour mixture over the prepared base.
Bake at in the preheated oven for 5 to 8 minutes or until top edge of crust is golden. Reduce heat to 100 C / Gas 1/4 and continue baking for 1 hour. Remove cake from oven and cool in tin for at least 3 hours. Remove sides of tin to serve.