Treadigraph, I cannot personally recall having heard anything about an Mi-2 over south London but there are doubtless others who may.
What I do remember (and I think it was early 70's) was that our Russian friends brought across an Mi-10 (the one capable of carrying a bus beneath it) and solicited a number of prospective clients including Bristows.
Sadly, I've been unable to dig-up anything about it.