Hi I've just finished restoring my nc29 took it for its mot and it failed on the lights dulling opposite to the indicators I thought it was an earth been through all of them ,New battery etc ,When I hold the revs at about 4 grand it disappears any ideas maybe rectifier?

Could be, have you tested the voltage across the battery at tickover and then around 3k to see if you are charging properly?

even with a good reg/rec the battery can be on its way out, is it very old?

It's a new battery but I haven't charged it what should the voltage be at tick over and 3k iknow everthing settles down around 3/4 k