An old friend of mine who was Indian (well, come to think of it, still is I suppose), said the only place he would buy Indian food was Southall.
More southerly and somewhat downmarket compared to Southall, Plumstead is very good.
If you eat Indian food at a wedding or at someone's house, the food is much better than the average restaurant but similar to what you get at the above mentioned places.
We have takeaways from the 'Yak & Yeti', which serves Indian and Nepalese cuisine. They have three restaurants in various places in London and the food is excellent.