Preheat the oven to 180oC, gas mark 4. Grease and base line a 21cm cake tin.
Place the dates and bicarbonate of soda in a bowl with 150ml boiling water and leave for 10 minutes. Blitz with a stick blender until smooth.
Meanwhile, whisk the butter and sugar together until pale and fluffy, whisk in the eggs, 1 at a time. Stir in the date mixture followed by the flour and then the walnuts. Spoon into the prepared tin and bake for 50-55 minutes until a skewer comes out clean. Allow to cool slightly before removing from then tin.
Place all the toffee and walnut sauce ingredients in a small saucepan and cook until bubbling and the butter has melted, allow to cool before spooning over the cake, allowing some to drizzle down the sides.
Cooks tip
This will keep for 2-3 days, great served warm with ice cream.