Put all the ingredients in a heavy-based pan and cook over a very low heat, stirring constantly until all the sugar has melted. You must stir constantly, otherwise the sugar will burn and the sauce becomes inedible.

Slowly bring to the boil, and simmer very gently for 20 minutes, continuing to stir all the while.

Remove from the heat and beat well with a wooden spoon.

Serve warm with a really good vanilla ice cream.