While I was trying to work out what was going on with the sidestand switch the other week, I had the bike on a rear stand and was checking that everything was playing ball as regards the sidestand switch and clutch switch affecting the bike being able to run or not. I noticed that if I put the bike in gear with the engine running and the clutch lever pulled back to the back, the rear wheel would rotate. The slighest hint of rear brake would stop it, and there was no drop in engine note, so there can only be the barest minimum of drag there. It would stay stopped afterward, unless I gave the bike a few more revs in which case it would start once again.

The cable adjuster is pretty much on the limit at the bars and the free play is probably a little more than the spec says it ought to be. Is this normal (I've never actually done this before in the 10+ years I've had it) or do I either need to see if I can adjust the cable from the other end? Presumably if this is an issue and adjustment doesn't have any positive effect, it will only be a matter of time before the clutch doesn't disengage at all?

Perfectly normal for the rear wheel to rotate slightly whilst in gear and engine running and clutch to the bar.
Easily stopped by hand.
The discs are fully apart but the oil between the plates creates the very slightest drag hence the shaft rotation.
This goes for pretty much all oil clutches.
Even air cooled ones usually have a place where one plate touches another and carries the very slightest of drag.