Thanks for the excellent report MTF, I'm very glad to see you back!
I've been to three "fancy" Indian restaurants in London, Tamarind, Zaika and Benares and they're all very different. Tamarind is located in a basement, the room is a bit crowded and hectic and we had to wait at the bar for our table, the place is indeed very popular. The food they serve is prepared and presented more authentically than at the other two places, without a hint of "fusion". It's been a while since I had dinner there, I remember the food being very, very good with a ginger lamb dish being outstanding and worthy of the Michelin star they still retain. Portions were small though and I remember thinking it was very expensive, even in relation to the quality.
At around the same time, I had lunch at Zaika, I think it still had a Michelin star back then. The room is really beautiful and wasn't very crowded at lunch time. We came in a little bit late for lunch and the staff seemed quite annoyed and service was accordingly bad. The quality of the food wasn't as uniformly good as at Tamarind, one dish was just OK while the other was world class. It's certainly the most "fusiony" of the three, especially the way the appetizers were presented. If anybody plans to go, I can highly recommend their duck in black lentils, possibly the best Indian dish I've ever had.
Finally Benares, it was recommended by locals who thought it was better than the two others. I remember the food only being so-so, with one appetizer being completely trivial and one main that consisted of nothing but big chunks of tender but completely overcooked lamb. Both came highly recommended by the slightly pushy waiter. The interior is really nice though, very modern and stylish, you wouldn't think you were sitting in an Indian restaurant. At least at the time I was there, it was was a very "in" place but the food was a letdown and the prices comparable to those at Tamarind and Zaika.
I'd certainly go back to Tamarind, probably to Zaika as well but not to Benares.