So, I removed the footrest nuts and am able to wiggle the battery
box up and down. Restarting the engine leaves me with the same problem. I'll state what I have done so far, in addition to above:
Thank you all for your suggestions
a) I took the clutch apart - all was very clean and dry (no oil oozing into the clutch) and all spindles are fine. I measured the clutch springs and they match up well. I adjusted the clutch cable as per the handbook instructions. With wheel off the ground and engine running, the rear wheel rolls to a stop when in gear and clutch (handlebar) lever pulled in.
b) Took the outer gearbox cover off - again. Minimal adjustment made to position of G61as per Richardson's book. Adjusted, slightly, the cam (G57) to allow more room by G49 for upward movement of the lever. Found there was a washer under the bolt head (right hand 327) which might have interfered with upshift and removed it.
c) Return springs (G64, G70 and G71) had been replaced with new ones a couple of hundred miles ago and all looked to be in great condition
d) The battery
carrier tray is a nice sliding fit with the two slim nuts up against G50. I don't know if that is enough.
e) I drained the gearbox of oil - beautifully clean without evidence of metal fragments, so replaced it via a filter
After all above, I replaced the cover, kickstarter lever, gear selector lever, G81 and readjusted the clutch. I can select all gears, up and down while the engine is off. Started her up and it appeared as if I'd done nothing - same refusal of the selector lever (G78) to even budge in upward direction!
I'm flummoxed, especially since all worked beautifully a couple of months ago.