Grease a 1kg rectangular terrine mould with butter.
Pour golden syrup into the mould and top with a layer of pear slices.
Beat sugar and butter until fluffy, then beat in eggs and fold in flour.
Spoon the sponge mixture into the terrine and smooth the surface.
Cover with greaseproof paper and foil tied securely with cook's string.
Place the terrine in a saucepan with water reaching halfway up the sides.
Simmer the pudding for 1.5 hours until a skewer comes out clean.
Simmer wine, spices, whole pears, and citrus zest for 12-15 minutes.
Boil milk, cream, and vanilla seeds in a small saucepan.
Whisk sugar and egg yolks, add hot milk, then thicken over gentle heat.
Allow pudding to cool slightly before inverting onto a plate to unmould.
Serve sliced sponge with spiced pears and warm custard.