Prepare the Pudding Basin:

Generously butter a 1.5-pint (approximately 850ml) pudding basin. Pour the golden syrup into the bottom of the basin.

Make the Sponge Batter:

In a large mixing bowl, cream together the softened butter and caster sugar until light and fluffy.

Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next.

Stir in the vanilla extract.

Gently fold in the self-raising flour, adding the milk one tablespoon at a time until the batter has a soft, dropping consistency.

Assemble the Pudding:

Spoon the batter over the golden syrup in the prepared basin, smoothing the top.

Cover the basin with a piece of buttered foil or a lid, ensuring it's tightly sealed to prevent water from entering.

Set Up the Slow Cooker:

Place the pudding basin into the slow cooker.

Carefully pour boiling water into the slow cooker until it reaches halfway up the sides of the basin.

Cover the slow cooker with its lid.

Cook the Pudding:

Set the slow cooker to high and cook for 3½ to 4 hours, or until the sponge is well risen and a skewer inserted into the center comes out clean.

Serve:

Carefully remove the pudding basin from the slow cooker.

Let it stand for a few minutes, then invert onto a serving plate.

Serve warm, ideally with custard or clotted cream.