Thank you both for your replies. As I always I am very grateful.
I've ordered a new gear change assembly and will install as soon as it arrives. I guess there is a chance that alone will make a difference, but I suspect something more will also be required.
Symptoms are:
It appears to be shifting from third past second to actual neutral. If I stop at that point, the neutral light is on and the bike rolls with the clutch out.
If I shift up again from that neutral position it engages second.
When the bike goes into neutral after downshift from 3rd to 2nd, the bike will not shift down to first from there. It will however shift easily back up to 2nd, after which it will shift down to 1st normally.
My workaround while riding for now is to downshift from 3rd to neutral, and them immediately up to 2nd and then back to 1st when stopping or otherwise requiring 1st gear.
Neutral can be a bit difficult to find from 1st gear. It's easier to go up to second and then move back down to neutral
Haggis, your suspicion regarding the selector forks is probably correct. Based on the symptoms do you still have that sneaky feeling? If so, do you have a hunch about which fork is most likely to be bent?
Cheers