Thanks for update – Ijunked the electro mech RR in favour of a solid state one as part o the rebuild so I don’t think this is the cause.
Battery is fully charged and the bikes runs great without the lights on (surprisingly well seen as the engine came semi dismantled in a box of bits!)
Charge light goes out and alternator is obviously charging.
I’m thinking that I might have a wire crossed over at the main switch as I used a wiring diagram that showed an unusual connection for the lights (ie different to the pukka MZ schematic).
…a job for the weekend!