Remove the stalks from the mushrooms and cut in half. Place in a small pan with the soy and 100ml water and cook for 5 minutes. Remove and drain on kitchen paper. Place the rice in a heavy-based pan with a lid. Cover with cold water and wash the rice, swishing it with your hands, until the water is cloudy. Drain the water and repeat until the water is almost clear - probably about 5 - 6 changes of water. Once the water is almost clear drain and add ½ pint water to the pan. Sprinkle in a little salt, put the lid on and bring to the boil. Once the water is boiling turn the heat down to the lowest level and cook for 10 minutes, then remove from the heat and leave to stand, with the lid on for another 10 minutes before lifting the lid. While the rice cooks, stir together the rice wine vinegar and caster sugar until the sugar dissolves. Fluff the cooked rice and turn out to a large bowl. Cool for a moment before stirring through the sushi vinegar. Form into small lozenge shapes with wet hands, dot each with a little wasabi if you like and top half the lozenges with piece of red pepper and the other half with the mushrooms. Serve with soy sauce to dip.