Many thanks guys,
jnanian: somehow that didn't turn up when I searched, but its bookmarked now! Thanks for the links.
siriusglass: Thanks for your suggestion. I took the front and rear elements off, and took the front plate and used some PTFE spray (Rocol Ultraglide) to lubricate the mech. As you suggested, I then worked the shutter at all the speeds many times. Some of the PTFE worked its way onto the aperture and shutter. When I cleaned the aperture, there was quite a bit of black that came off on the wipe. It eventually cleaned up AOK though. Likewise on the shutter blades (very careful here!), careful cleaning and repeated working of the blades and cleaning in cycles brought them up clean.
So, the good news is that it now seems to be working perfectly!! All speeds seem correct, all workings are reliable, and the adjustments seem smoother. I need to do more checks over the next few days, but hopefully it will be OK. It was quite cold today when the problem surfaced, so a spell in the fridge and more checks would be in order I think.
PS thanks to the poster on another thread that reported the ptfe spray working well for him.