Thanks everyone for your suggestions, would have replied sooner but the post notifications aren't arriving in my email as they should
Will try the Notify me of replies box again.
Ok - bike is +ive earth. What I mean is the ammeter shows the correct charge when the bike is started and no lights on (just above halfway). With the lights on, the ammeter dips initially then returns to roughly halfway. When it's revved, the ammeter returns to the centre and will not move under charge and no lights, horn, brake light work.
I know there is charge in the battery and that it's functioning correctly because, occasionally, everything works. Sometimes nothing comes on and then, miraculously, after a few seconds, everything comes on again.
The bike had a brand new loom on rebuild.
I think I've got to pull it apart a bit to see if I can identify the problem - as most of you suggest. It's just a bit of a tits up as I don't have an obvious starting point.
Thanks again folks - suggestions, advice and knowledge always welcome!!!