Grease a 1.5-litre pudding basin well with butter.
Pour the golden syrup into the bottom so it coats evenly, this will become your molten topping when turned out.
In a large bowl, combine the flour, suet, spices, sugar, dried fruit, lemon and orange zest, and salt.
Stir in the beaten eggs and just enough milk to bring it together into a thick, dropping consistency. Don't overmix.
Spoon the mixture over the golden syrup, smoothing the top.
Cover the basin with a pleated sheet of baking parchment and foil (the pleat allows for expansion). Tie securely with string.
Place the basin in a large pot. Pour in boiling water to come halfway up the sides.
Cover with a lid and steam gently for 2–2½ hours, topping up with more boiling water if needed.
Carefully remove, rest for a minute, then invert onto a warm serving plate. The golden syrup will cascade over the pudding.
Cut generous wedges and serve with lashings of custard.
