Many thanks gents for contributing help to this. I know its an old chestnut and a blindspot (no pun intended) for me.
I'm going to get one of the Xenon H4 bulbs and give it a go and then look to upgrade the alternator and Rob Lafferty has said he will look into modifying the wiring loom or switchgear.
Debbens do a higher output alternator for J and GPz if all else fails!
Scroll to the bottom of this page
http://www.debben.co.uk/acatalog/Z1000J.html
The Xenon micro bulbs look like a possibility too.
I'm certain that taken together with the mods already done to the wiring that this should put an end to it for me.
Once again thanks.
AL