Thanks again for the advice everyone, sorted the problem but I must confess I feel like a bit of a knob! Here’s a quick summary in case it helps someone else.
Finally got some time to look at the bike Saturday morning. Inspected the clutch hydraulic system. I noted fluid was a little low but the lever moved smoothly with constant hydraulic pressure throughout the range of motion (no slack early on). Fluid was starting to darken, so flushed and bled the system with fresh DOT4 anyway. I also took the oil filler cap off so I could see the clutch plates move in response to lever. Outside plate moved approx 2mm, so action looked ok.
It was then that I noticed the gear change lever! It was bumping the side stand when trying to shift into first… yep, the locking nuts had come loose on the adjuster shaft, which allowed the lever to slowly drop over time!
So I’m relieved to report a simple and free fix. I’ve been riding and tinkering with bikes since I was young, so also highly embarrassed to have overlooked such a fundamental adjustment!
Still have some drag on the clutch, and gear changes are a bit rough so I'll continue and take a look at the clutch plates...