Grease a 1 litre/2 pint pudding basin with butter. Pour the golden syrup into the basin.
In a mixing bowl, cream together the butter and sugar until light and fluffy (a handheld electric whisk works best). Stir in the lemon zest.
Beat in the eggs, one at a time, alternating each egg with a tablespoon of flour. Fold in the remaining flour, lemon juice and milk and pour into the basin, covering the syrup.
Cut out a circle of greaseproof paper one and a half times as large as the diameter of the pudding basin. Fold a pleat along the middle of the greasproof paper and place over the top of the pudding basin. Tie securely with a piece of string and tie a separate loop of string across the top to form a handle. Trim off any excess greaseproof paper.
Place an upturned saucer into the base of a large lidded saucepan. Sit the basin on the upturned saucer and carefully fill the pan with boiling water until it comes two-thirds of the way up the sides of the pudding basin. Make sure the water doesn't come into contact with the greaseproof paper, which could make the pudding soggy.
Place the lid on the pan, place over a low-medium heat and bring the water to a gentle simmer. Steam for one-and-a-half hours, checking the water level from time to time and topping up with more boiling water if necessary. (Do not lift the lid for the first 30 minutes of cooking or the sponge may collapse.)
Carefully lift the pudding basin out of the pan using the string handle. Remove the greaseproof paper, gently run a knife around the edge of the pudding basin and turn the syrup sponge out onto a serving plate. Serve at once.
