Where else in London (or England) can you eat genuinely good food for around £12? This café does not take bookings and has fairly basic decor (no tablecloths) but the service is efficient and you can buy alcohol from the shop two doors along (Diwana's is unlicensed) though the lassi here is excellent and refreshing. The south Indian vegetarian food here is excellent if you order carefully.
Diwana is best at the starter snacks, so I normally order two or three of these per person, and then just skip to the home-made kulfi (Indian ice cream). You should try the bhel poori, samosas and aloo papri chat, which are outstanding. The bhel poori itself has a wonderful balance of flavours and textures, the spices immediately apparent as the first bite touches your tongue. The blend of textures and spices in the bhel poori in particular is excellent, with little pieces of poori, potato, coriander and tamarind sauce. The samosas here are generous in size, bursting with mixed vegetable filling and served with a spicy tomato sauce.
The de-luxe dosa (basically a large crisp pancake stuffed with a spicy potato and onion filling) is also excellent, though the main course curries are rather ordinary; they can be a little watery and dull, though both chapatis and parathas are fine. They offer an "eat as much as you want" lunch for £6.50; this is excellent value, but does not usually include the snacks at which they are best. In addition to the excellent kulfi, they also have the rich, comforting dessert called shrikand, here quite thick in texture with a hint of saffron. Few restaurants in the UK make this dish.
The only reason I do not score this higher is that if you stray from the snacks on to their main course curries you will be disappointed; these are very ordinary indeed. But if you stick to the sort of dishes I describe then you will have a meal that is easily 12/20 level (and higher in places), yet at such a low price that you have to pinch yourself.