Mix the flour, baking powder, suet, sugar, currants, and lemon zest in a bowl.
Stir in the milk to form a soft dough.
Spoon the mixture into a buttered pudding basin and cover with folded greaseproof paper.
Secure the paper and a damp tea towel over the basin with string.
Place the basin in a large saucepan and fill two-thirds height with water.
Cover the pan, bring to a boil, and simmer for one hour.
Simmer the milk and cream in a saucepan for the custard.
Whisk the egg yolks and sugar together until light and frothy.
Slowly stir the hot milk into the eggs, then thicken over low heat.
Slice the pudding and serve immediately topped with custard.