Preheat the oven to 160°C, gas mark 3. Grease and line a 900g loaf tin with baking parchment. In a large mixing bowl, use electric beaters to cream the butter and caster sugar for 3-4 minutes, until light and fluffy. Beat in the eggs one at a time, then beat in the almond extract.
In a separate bowl, combine the flour, salt and almonds, then fold into the wet ingredients until combined. Stir through the grapes, then spoon the mixture into the loaf tin, levelling the top. Bake for one hour to one hour and 10 minutes, or until a skewer inserted into the centre comes out clean. Cool in the tin for 20 minutes, then carefully transfer to a wire rack.
As soon as the cake is on the wire rack, place the halved grapes in a small roasting tin with two tbsp water. Roast in the oven for 20 minutes, until soft but still just holding their shape, then remove and lift the grapes onto a plate. Add another 1 tbsp water to the roasting tin and scrape up the inky grape juices with a wooden spoon. Adding a little at a time, stir in the icing sugar with a wooden spoon until you have a spoonable icing. Spoon over the cake and top with the roasted grapes. Cool completely before serving.