Grease a 1 litre pudding basin with butter and pour in the golden syrup.
Cream the butter and sugar in a bowl until light and fluffy, then stir in lemon zest.
Beat in eggs one at a time with a tablespoon of flour, then fold in remaining flour, juice, and milk.
Pour the batter into the basin over the syrup.
Cover the basin securely with pleated greaseproof paper and tie with string to create a handle.
Place the basin on an upturned saucer inside a large saucepan and fill with boiling water to two-thirds depth.
Steam for 90 minutes over low-medium heat, topping up boiling water as needed.
Remove the basin, loosen the edges with a knife, and invert onto a plate to serve.