Pre-heat the oven to Gas Mark 4, 180°C, fan 160°C. Grease and line 2 x 7 inch springform cake tins with nonstick baking paper. Combine the self-raising flour, butter, eggs and sugar in a large mixing bowl for the cake batter. Beat using an electric hand-held whisk for 2-3 minutes until smooth. Dissolve the coffee granules in a little hot water to make a thin paste. Pour into the batter and mix well to incorporate. Spoon into the prepared cake tins and bake for 25-30 minutes until risen; remove when a cake tester comes out clean when inserted into the centre of the cakes. Allow the cakes to cool a little in their tins before turning out onto a wire rack and removing and baking paper. Prepare the icing for the filling as the cakes cool. Mix 1tsp of the instant coffee granules with 50g of the icing sugar in a mixing bowl. Add a little hot water, 1tsp at a time, and beat until you have a spreadable icing. Spread onto the top of one of the cakes and place the other cake on top, pressing down gently, to sandwich. Prepare the icing for the top by beating together the remaining icing sugar with the unsalted butter until smooth in a mixing bowl. Dissolve the remaining coffee granules in a little hot water to make a thin paste and add in batches to the buttercream, beating well between additions until you have the desired coffee flavour. Spread on top of the cake in an even layer, then pulse the walnuts in a food processor for a few seconds until finely chopped. Sprinkle on top of the cake so that they stick to the icing. Cut into portions and serve on plates.