One fairly good place for Japanese sashimi and grilled fish is The Ikkyu Japanese restaurant at Tottenham Court Road 67A. Nearest tube stops are Goodge Street, Tottenham Court Road and Russell Square. It's down in the basement so a little bit hard to notice from the street. Food is authentic Japanese and many of the habitues Japanse professionals and academics. Please don't mix with the far less desirable Chinatown offspring.