Well, I've had the unit dismantled, and managed to again get it mantled (is there such a word?)......no sign of wear or any breakages, so it may well indeed be just the grease thickening up- which is no reassurance, really- if I'd found something obviously snapped or mangled, fair enough, but its worrying if something stops working without some kind of spectacular failure event!!
One thing I do notice, now that its 'working' again, is that the lever seems to 'lose' the ratchet at the ends of the shift range. Having got a good look at the components and how the thing works, it seems to me that the weakness is in the secondary lever- the torsional return spring arrangement seems to be a bit hokey, as is any method of keeping the lever well fixed and under tension into the body of the mechanism, which I suspected was the problem at the outset- it looks like it can easily be tweaked and modified using bits from an old floppy disc to function reliably- pity Shimano didn't pay more attention to this in their R&D of the mechanisms.
I will go with Retro Grouch's 'Low Maintenance' concept and the bar end shifters, though- I don't fancy riding with the Gear Lever equivalent of the British SA-80 rifle!!
Cheers, and thanks for all your help,
Dav
PS- Hard Cheese for us Brits in the Olympic Keirin- Dangerous Riding!!? I thought the whole concept of the race was kind of a Velocipedal Rollerball...........