posted this in the duff battery thread as an update..
Okay some testing done...
1. Dynamo drive/chain ok no slip or breaks
2. Dynamo wiring/brushes etc a ok no issues
3. Dynamo output 20+ volts rising smoothly(tested at regulator connections) its a 12v armature
4. Brown wire from regulator to ammeter/battery etc disconnected and tested
5. 4 volts max rising smoothly to 4v but no more
6. Red earth cable from regulator tested with audible tone a ok
7. ? This reg is brand new so it shouldn't be a problem the last one lasted 5 years until this exact same symptoms happened. Sorry but it has to be said WTF.
I am going to send the reg for testing as a double check and go from there. I will use the bike for work as its daylight only and now with a new battery fitted I will have brake lights.