Preheat the oven to 180C/160C Fan/Gas 4. Grease a 900g/2lb loaf tin and line the base and ends with a strip of buttered baking paper.
Combine the butter, dates, boiling water and instant coffee granules in a large mixing bowl. Mix to combine and leave for 30 minutes for the butter to melt and the dates to soften.
Add the caster and dark muscovado sugar to the date mixture, stirring well to combine and to dissolve any lumpy brown sugar. Add the beaten eggs and mix again to combine. Sift the flour, baking powder and a pinch of salt into the bowl and mix until smooth. Add all but 1 tablespoon of the chopped walnuts and mix again. Scoop the mixture into the prepared tin, spread level and scatter the remaining walnuts and demerara sugar over the top.
Bake on the middle shelf of the preheated oven for 30 minutes, then reduce the oven temperature to 170C/150C Fan/Gas 3 and cook for a further 25–30 minutes, or until a wooden skewer inserted into the middle of the cake comes out clean. If the cake is browning too quickly, loosely cover the top with foil or baking paper.
Remove the cake from the oven, leave in the tin for 3–4 minutes, then carefully lift out of the tin and leave on a wire cooling rack until completely cooled.