Put the apples, cinnamon, lemon zest and juice and sugar in a saucepan.
Cover and simmer gently until pulpy, stirring occasionally.
Beat thoroughly with a wooden spoon, then cook, uncovered, over a high heat, stirring continuously, until any excess liquid has evaporated and the purée is very thick.
Brush the butter all over the slices of bread.
Line the base and sides of a greased 6 inch deep cake tin (such as a Christmas cake tin or substitute a coffee can or vegetables tin, as long as it does not have ridges) with the slices of bread, making sure that they overlap.
Spoon in the apple purée and cover with more over-lapping slices of bread.
Bake for about 30 minutes, until the top is golden brown.
Serve at once, turned out and accompanied with real custard or fresh cream or clotted cream to pour over it.