Just a quick update to anyone that is interested. In the end I decided on the QL17 despite the problems I had with one I tried before due to it being much smaller than a GSN etc. Got one off ebay for probably a little over the odds but it had been fully checked out with replaced seals, when it arrived it looked basically new, ready to give another 20+ years of service!
Took it out to London street shooting and it's small size and quietness made quite a difference. Still not sold on rangefinder focusing, I find it a bit slow and difficult compared with a manual focus slr but I'll get used to it. Didn;t have any real issues adjusting teh apature and shutter speeds as I did before, I think the one I tried had much stiffer rings.