Well, I managed to get the top off the GS645 over the weekend - the screw holding the wind-lever in place has a standard (right-handed) thread. The rangefinder is now adjusted at infinity :-)
Unfortunately, I managed to drop the camera whilst out-and-about with it on Saturday afternoon, so it has also been undergoing some reconstruction this weekend (cracked plastic around the viewfinder and split 'flexible PCB' caused by the impact - had to run some extra wiring inside to re-join the cut tracks). Needless to say, I wasn't happy about my stupidity :-(
Having said that, it all seems to be working again now...