Peel, core and thinly slice the apples first of all, rinse them in cold water and put them in a saucepan with the sugar and 1 oz (25 g) of the butter.Cook them over a low heat until they are soft enough to beat into a purée. Beat them and leave on one side to cool. Meanwhile melt the remaining 3 oz (75 g) of butter gently, and cut each slice of bread into rectangles. Next brush each piece of bread with melted butter (both sides), being careful not to leave any unbuttered patches, then line the pudding basin with approximately three-quarters of the bread (or as much as you need). Don't leave any gaps between the pieces – overlap them and press firmly.When the apple purée has cooled, beat the egg yolk into it and fill the lined basin with the mixture. Finally seal the top with overlapping slices of the remaining bread. Place a suitably sized ovenproof plate on top of the pudding and weight it down with a 2 lb (900 g) scale weight. Meanwhile, pre-heat the oven to gas mark 6, 400°F (200°C).After 30 minutes place the basin (with the weight still on it) in the oven to bake for 35 minutes. Then, with an oven cloth, remove the plate and weight, and bake the pudding for another 10 minutes to brown on top. Leave the pudding to settle in the basin for a minute after removing from the oven, then carefully invert it on to a warmed plate to serve.