Preheat the oven to 180C/350F/gas 4. Peel, core and thinly slice the apples into a bowl. Mix in the demerara sugar, lemon zest and juice and the cinnamon stick (broken into 5 or 6 pieces). Make the buttered crumbs by melting the butter in a heavy frying pan and tipping in the breadcrumbs. Cook over a medium heat, stirring constantly with a wooden spoon until all the butter is absorbed into the bread and the crumbs have separated out and are a golden colour. Layer the apple and crumbs in a buttered ovenproof dish (1.25 litre/2 pint size) finishing with a layer of crumbs. Press down with the back of a spoon. Sprinkle the extra demerara sugar on top. Bake in the centre of the oven for about 45-50 minutes until the apples are soft (test with a skewer) and the top is crispy and brown. If the pudding seems to be getting too brown before cooked, cover the surface lightly with a piece of baking parchment or foil, but dont tuck it in as it will spoil the crispy surface. Serve with clotted cream.