To make the sauce, combine the sugars, salt and water in a saucepan.
Stir until dissolved, brushing the sugar crystals from the sides of the pot if needed. Bring to the boil, and continue cooking, without stirring, over medium heat for around 10 minutes until it reaches hard ball stage (122°C on a candy thermometer) or is frothy on the top with large bubbles around the edge. Add the butter and continue to simmer for about 5 minutes, swirling the pan from time to time, until it reaches hard crack stage (154°C) or is very frothy and smells of almost-burnt caramel. Carefully add the cream a little at a time, stirring, until well combined and smooth. Remove from the heat and pour over the hot pudding as soon as it is removed from the oven.