Grease a 1.7-litre pudding basin and carefully line the bottom and sides with the clementine slices.
Put the marmalade in a small pan with 2 tablespoons of water and bring to the boil, stirring. Pour into the prepared basin.
Using an electric hand whisk, cream the butter and sugar for 3 minutes until light and fluffy, then whisk in the eggs.
Sift over the flour, baking powder and bicarbonate of soda, then fold into the creamed mixture until just combined.
Gently stir in the milk. Spoon the mixture into the basin.
Take two pieces of baking paper and one piece of foil (all large enough to easily cover the basin) and put on top of one another, with the foil on top. Fold a pleat into the stack of papers (this gives the sponge room to rise), then grease the bottom sheet of baking paper with butter. Cover the pudding, buttered baking paper side down, and tie tightly with kitchen string around the rim to secure.
Transfer to a large pot, in which you’ve placed an upturned plate. Pour boiling water halfway up the pudding basin. Cover with a lid and steam for 1 hour 45 minutes. Lift out the pudding basin, then carefully turn the sponge out onto a plate. Serve with the warm custard.