I am a Japanese national, but lived in UK for a year and ate at various Japanese restaurants here. I dined at this place several times, and it used to be a fairly good Japanese diner with inexpensive, everyday foods one eats in Japan. But this August, I am very disappointed. The rice is no longer Japanese type: it is long grain and not very white. We ate Teriyaki Chicken but it is just a chicken cooked with a soy source: it is not "Teriyaki" at all with the characteristic shiny surface, which "Teri" means. Some pieces of fried potatoes came with the dish, and they are very hot with red pepper. Japanese foods do not use much hot pepper. I think the cooks there are either no longer Japanese or trying to cater, not authentic Japanese food lovers but to those who are only curious about Japanese foods. As if to prove this, most of the customers who were there when I ate were not Japanese. A sign of good foreign restaurants is that there are many nationals of the country from where the foods come.
That said, Tokyo Diner is still a reasonably priced, conveniently located East-Asian restaurant. If you are not particularly fussy about eating usual Tokyo foods as I am when I go to a Japanese restaurant, I can recommend it. It is so much better than ubiquitous Wagamama.