[FrankenDesmo]

Aye, it definitely seemed to be a sudden issue that had developed while sitting over the winter (rather than incorrect jetting). The battery was new on the bike when I got it, but it hasn't been replaced since the RR was replaced (it holds a strong charge though, and was off the bike and kept topped-up with a trickle charge over the winter). I'm hoping it was old fuel gumming up the works, but I'll see when I go to start it again.

However, I've just done the valve clearance checks...

Vertical Inlet Open: 0.04mm (0.05 required a tiny amount of force)
Vertical Inlet Close: 0.14mm
Vertical Exhaust Open: 0.04mm
Vertical Exhaust Close: 0.09mm
Horizontal Inlet Open: <0.04mm (didn't have a feeler gauge smaller, 0.04 required a tiny amount of force)
Horizontal Inlet Close: <0.09mm
Horizontal Exhaust Open: <0.04mm (same as Inlet)
Horizontal Exhaust Close: <0.09mm

I used the 'loaded gap' method from Chris Kelly to measure the Closer gaps, so the <0.09mm on the horizontal Closers means 0.13-<0.04, so probably 0.10 assuming the Openers are 0.03mm.

I triple-checked these measurements, going up and down feelers (where possible) to ensure I had the one that slid in without force. Where I need to apply a tiny amount of force, it was really minute and more aligning the gauge with the gap by using a finger at each end of the gauge, but I assume that counts as the gauge being just too thick.

My Closers seem to be within spec (0.03-0.20 according to Haynes), however it looks like all four openers are out of spec as Haynes has the lower bound at 0.05mm.

I gather that this means that the Opener shims need to be ground down by 0.01-0.02mm, in order to open the gap a bit.

Bugger, I was hoping to not have to go that far just yet. Still - could be worse. Off to machine mart to get a micrometer, some 400 & 600 grit, and a flat surface.