Hi John,
Hope I'm not too late replying, but I had the same problem on my mk1.

As it warmed up the lights would start to dim and the rc valve would slowly work it's way round until it was fully open. It was putting out about 4.5 volts.

Suspected the stator, and after a chat with West Country ...

Thanks, Eddie, for a brilliant weekend. I'll be suffering tomorrow!!
It was awesome, riding in the group! Not a 4 stroke in sight ! (Though there was a bit of blue !)
The smell of 2T on the way back from the Ponderosa was a real 'Condor!' moment, until I was
woken by that copper overtaking me ...

Trashed my lower seal on my mk1 when I did the head bearings.
Both upper and lower seals look to be the same.
While your at it, it's worth drilling and tapping the headstock, and fitting a grease nipple.
Though, if like me, you don't replace the seal, the grease works it's self out, landing on the ...