quilon and the bombay brasserie are very good. they are both run under supervision of the taj group, which means that the chefs get trained properly. they know what the finished product should be like, and they've been trained to deliver it. the bombay brasserie is the one restaurant in london where you can see vindaloo and rogan josh together without shuddering; quilon serves impeccably made keralan. having said that, be warned that there's an element of soul missing - you'll get great technique, never an inspired sloppiness.
they aren't cheap, these restaurants, but they are a mile and away in quality compared to almost anything else in london. they also let you get a chance to stray away from the punjabi/moghlai overloaded with cream curry sauces that every other restaurant seems to aspire to. but you'll pay up for the privilege at dinner; on the other hand, go to the brasserie for weekend lunch - its an amazingly good deal at £16.
in that vein, i should mention cafe spice namaste: parsi fare, not the standard rogan josh/chicken tikka masala. acceptable dhansaak, for example.
new tayyab isn't bad pakistani - but i'd say its dhabha quality, nothing like what lahori chefs are renowned for.
mela is pretty mediocre. (mela means fair, so i thought i'd get chaat and things. instead, its a mild variation on the usual tandoori routine - you get duck meat, whitebait etc.) diwana bhel puri house is atrocious, as are the rest of the offerings in euston. ditto southhall - never found anything there worthy of mention. chetana in wembley has an o.k.ayish thali, but its not worth the effort to get there.
then theres zaika, which i understand and nod my head to, but don't really dig. cinammon club is unspeakably bad, tamarind is even more expensive than the brasserie without being anywhere as good, chutney mary - god knows what chutney mary's upto - vama ehhh.