Preheat the oven to 160°C/325°F/gas mark 3 and grease a 1.6 litre deep Pyrex dish with butter. Tip the dates, bicarbonate of soda, grated apple and water into a food processor and blitz to a rough paste. Add the butter, sugars and eggs and blitz again until smooth.
Tip the mixture into a large Pyrex bowl and fold in the flour and salt. Pour the mixture into the greased Pyrex dish then bake for 1 ½ hours. To check that the pudding is cooked, insert a cocktail stick or skewer into the centre; it should come out clean.
For the sauce, combine the ingredients in a saucepan and slowly bring to the boil. Simmer for 5 minutes stirring constantly, until all of the sugar has dissolved and the sauce is smooth and glossy.
Turn the pudding out onto a serving plate and pour the sauce over the top. Serve with cream, clotted cream or ice cream.