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. Place the nori on a sheet of cling film. With dampened fingers press 1/6 of the rice over two thirds of the nori along the long side leaving about 1cm strip uncovered and sprinkle with some of the sesame seeds. Turn over so it is rice side down. Press a little of the wasabi in a line down the centre of the nori. Top with a line of avocado and salmon. Dampen the nori that isn't covered in rice and roll over the filling, then fill further to form a tube, pressing together well. To cut the sushi into pieces dampen a serrated knife. Cut each into 6 pieces. Serve with soy sauce to dip.