Lightly butter and flour a 1.2 litre/2 pint pudding basin.
With an electric hand whisk or wooden spoon, beat together the butter, sugar and lemon or orange zest in a large bowl until light and fluffy. Gradually beat in the eggs, adding a little flour as you go, to prevent the mixture from curdling. Mix in the remaining flour with the baking powder, breadcrumbs and 2 tablespoons of the golden syrup.
Put the remaining golden syrup into the base of the pudding basin and spoon the sponge mixture over, levelling the top with the back of a wet spoon.
Cover the basin with a double piece of aluminium foil or greaseproof paper, with a pleat across the centre to allow space for the pudding to rise. Secure well by folding the foil around the edges and tying with a piece of string.
Place in a steamer over boiling water for 1½ hours, topping up the water frequently. To check if the pudding is cooked, pierce a skewer into the centre of the basin. If it comes out clean, the pudding is cooked.
Turn out the pudding on to a plate, making sure you scrape out any sticky syrup and spread it over the top. Serve straight away with custard and an extra drizzle of golden syrup.