Mix the raisins, zest and juice together and leave to soak for 10 mins. Preheat the oven to 170C/Fan 150C/325F/Gas Mark 3.
Spread the bread with the butter and cut each slice in half diagonally. Beat the milk, cream, eggs, vanilla and 4 tbsp sugar together. Lay 4 slices of bread over the base of a 900ml/2pt shallow ovenproof dish. Arrange the remaining slices of bread on top, overlapping them slightly. Scatter over the raisins, zest and juice. Pour over the cream mixture and leave to soak for 15 mins. Scatter over the remaining caster sugar. Place the dish in a large roasting tin and pour boiling water from the kettle to come half way up the outside of the dish. Bake for 35-40mins or until the custard has set in the middle and top is golden brown. Serve hot or warm.