Using a rolling pin, crush one 100g bar of plain chocolate, still packaged. Open the package and empty into a pyrex measuring jug. Boil ~3 cups full of water and pour into a deep pan. Stand the measuring jug in the pan over the heat so that the water comes about halfway up the side. Add 1 rounded dessert spoonful of margarine or butter. Stir as the chocolate melts.
Dip another dessert spoon in the boiling water for about 20 seconds and then use the hot spoon to add 2 or 3 big dessert spoonfuls of golden syrup. The hot spoon should mean the syrup glides off easily.
When the mixture is smooth and uniform, leave to cool. The sauce is good hot or cold: whilst it thickens on cooling, it should not set (unless you haven’t added enough golden syrup) and can always be reheated.