Listen, you don't go to Diwana for the service. You go for the buffet. That being said I've been going for years and they are always very nice to me. Maybe because I'm friendly and I tip :)
Keep in mind the chef for the ala carte is different than the one for the buffet. The menu can be hit or miss in my experience but the buffet is amongst the best Indian in London. It's even better than the Indian's in India do it accordingly to my mate who spends a great deal of time photographing over there. Go on Fridays as they offer Matar Paneer. Also, insider tip: skip everything before the pomopadoms. The real indian food starts afterwards.