Put 100g cranberries in a saucepan with the redcurrant jelly and sugar. Simmer for 5 mins, then leave to cool. Meanwhile, put a pan of water on to simmer and butter a 1.4 litre/2½ pint basin. Butter a large sheet of baking paper, sit on a same-size sheet of foil and fold a leat in the middle. Beat all the sponge ingredients together, then stir in the remaining cranberries. Spoon a third of the cranberry and redcurrant mix into the bowl, then spoon the sponge mix on top.
Cover with the paper and foil, tie with string and steam, lid on, for 1½ hrs. To serve, reheat the cranberry mix and pour over the top of the turned-out pudding. Can be made up to 2 days ahead and re-steamed for 30 mins or microwaved for 5 mins on High. 3 To make the custard, put the chocolate into a microwave-proof bowl, add the custard and heat according to pack instructions, stirring every 30 secs until the chocolate has melted.