Thanks for your comments, everyone. The more I read, the more committed I am to getting the most from this camera. In a world in which there's a constant drive toward bigger and better, I think there's probably a lot to be said for exploring and making the most of what we've already got.
I had another film processed today - this time, with the lens locked at the extended length (!) - and they were perfectly focused and clear, I'm relieved to say. Again, there is no sign of the greying that I thought was the shutter mis-timing, so maybe that's not an issue. I'm also pleased (but slightly embarrassed) to say that the fogging of the rangefinder is now gone, having cleaned the glass on the rangefinder...
Would it do any good to post examples of works done with the M3 and other old Leicas, or would the quality of the images be lost on the web? Once I've got everything in place, I might try some comparative shots - the same subject with the M3 and my 40D with comparable settings - to see what happens. If it makes any (visual) sense, I'll post them. Does anyone have any that would show what their old Leica can do?
Edit - ps It also seems as if the Meter MC is working fine too - I bracketed either side of the aperture it was recommending, and the best shot was what the reading stated. Maybe it has lasted well because it has spent most of its life in the ERC/ dark?