Thanks for all the comprehensive replies!
I get the impression that some pilots think they might be in receipt of some sort of radar service when they are given a squawk for a FIS - as Evil J quite correctly states it is the pilot who is ultimately responsible for separation and perhaps it is timely to note that the legislation states that lookout must be maintained whether or not the aircraft is in receipt of an ATC service.
Finally, I am somewhat surprised, nay disappointed, that a minority of pilots seem happy to fly in cloud without any form of radar service - I suggest this is an airborne form of Russian Roulette.