Put 125 g (4 oz) of the sugar in a bowl with the flour. Warm the milk in a small saucepan on the hob and mix with the melted butter and egg yolks. Add to the dry ingredients along with the lemon rind and juice. Whisk well to mix.
Whisk the egg whites in a thoroughly clean, dry bowl until soft peaks form. Gradually whisk in the remaining sugar, a tablespoonful at a time, until the mixture is smooth and glossy. Whisk a quarter of the whisked whites into the lemon mixture to lighten it, then stir in the remainder.
Scatter the sultanas over the bottom of an ovenproof dish and pour the lemon mixture over the top. Place the dish on the lower rack of the halogen oven. Carefully pour hot water into the bowl of the halogen oven until it comes slightly up the sides of the dish.
Set the temperature to 175°C (347°F) and cook for 30 minutes until the pudding feels spongy on the top but is quite syrupy on the bottom. (Test by inserting the tip of a knife down the side of the pudding.) Serve with lightly whipped cream or crème fraîche.