Grease an ovenproof dish (approximately 1.4 litres/2½ pt). Cut the bread into even slices and spread one side with butter and then with lemon curd. Arrange the bread in the dish, butter side up, and sprinkle with the lemon zest and ginger. If using sultanas, first place them in a small bowl with the lemon juice and leave for a few minutes to absorb the liquid. Then sprinkle them over the bread.
Heat the milk in a pan and add the sugar, vanilla essence and egg yolks. Whisk continually over a low heat until the mixture shows signs of thickening, then remove from heat. Do not boil as the custard will curdle.
Sieve the custard into a jug and pour it over the bread, leaving it to soak for 10-15 minutes until the bread softens and absorbs the mixture. Place the pudding in a preheated oven @ 200˚C/Gas Mark 6 for 20-25 minutes until it sets and is puffed up and golden.
Dust with Demerara sugar and serve warm with yoghurt or crème fraiche.