Thanks for everyones help with this. It's been a bit of a challenge, but the wealth of knowledge here is invaluable.
So, you'll remember the steels in the clutch were quite corroded. I carefully got rid of that on them with some 0000 steel wool, and gave those and the friction plates a real good scrub down with Brake Cleaner. Left the cleaned friction plates soaking in oil overnight and then rebuilt and refitted the clutch, a new gasket on the clutch cover, and then with my heart in my mouth this morning I got the bike up to temperature, squeezed the clutch many time, and then dropped it into gear. Lo and Behold, all is well. Clutch seems to be operating beautifully now and the lever feels as I would expect it to feel. The tips regarding adjustment were invaluable. But now I have no drag, and I have all my fingers crossed.
One little thing, I really can't get the thing into neutral with the engine running, but I'm thinking this is just a matter of more trial and error adjustment??
After some experimentation I seem to have found the shortest route for my short clutch cable, and at least now it's not stretched like a guitar string on full lock. I'm going to spend some time looking around for the appropriate cable (Thanks for the tip regarding the 500 cable, Johnny), however I'm more and more attracted by the thought of the lower European bars... that seems like a good solution, and I actually find these bars a little tall anyway. It's just a matter of sourcing said bars now. I can find pattern 'drag' bars to fit, but I'm guessing they will be missing the vital cable entry holes. Any ideas much appreciated.
Once again, Thanks All. Much appreciated.
T