It was possibly off Ghee (clarified butter), that can go rancid and taste like off milk. Or it might have been the gravy they used, left to stand too long.
I am not fond of British Indian curries, I find the spices often taste stale, being an Aussie I am more used to the South East Asian style of cooking curry dishes. I do try to like curries, seeing as we have a curry house almost next door!!!
Funnily enough i saw the Ghee in the kitchen. I know the guy and he invited me into the kitchen to taste my curry to see if it was the right strength. When i tasted it there it was ok. I'll have a quiet word in his ear when i next go in.
Whereabouts in the UK are you kyles? portsmouth here.