Put the milk, cream, zest, vanilla pod and seeds, plus 50g sugar in a pan. Bring to a simmer, stirring often to dissolve the sugar. Take off the heat; stand for 5-10 minutes.
Whisk the whole eggs and yolks with the remaining 50g sugar. Strain the warm milk over the top, whisking to combine, then set aside for 30 minutes.
.Preheat the oven to 150°C, gas mark 2. Butter a 23cm x 23cm gratin dish; spread 3 tbsp marmalade over the base. Cut the ginger cake into 0.5cm slices; butter each as you go. Put buttered-side up in the base of the dish to form one layer, overlapping slightly.
Pour over the custard and press the cake mostly under the surface. Boil the kettle. Place the gratin dish in a large roasting tin and put in the oven. Before you close the door, pour boiling water around the outside of the gratin dish so that it comes halfway up the sides. Cook for 30-40 minutes, until the custard is just set.
Melt the remaining 2 tbsp marmalade in a small pan and brush over the pudding. Grill for 1-2 minutes, until bubbling. Rest for 5 minutes before serving, with cream, if liked.