Yes Hamish, the voltage sensing is provided via the kill switch and the injection relays circuit. Wiring it straight to the battery might not be a good idea or you could end up with a flat battery quite quickly when the ignition is off. Doing the wiring mod to bypass the ignition switch with a relay and wiring the voltage sensing to the relay output would be a lot better though.
Better still, throw the crappy regulator in the bin and fit a modern Shindengen MOSFET, serial or shunt, would still be an improvement and if you want retain the charging light fit an external warning light switch.

Cheers Guys, The problem with this electrical issue is that it has been on going for almost 2 years now and to be honest the nature of my work means weeks away and sometimes a month or two before even looking at the bike.

I had in the past done the various tests and all things rung true the original fitted rectifier was replaced with a direct compatible replacement at great expense to resolve an earlier charging issue that was eating up batteries while out on a run and to be honest seemed to be a fix for a few months?
The second replacement was a cheaper but compatible replacement that I was going actually use to suss out the electrics on the 906 rebuild I have on the go.

I will at the weekend double check the Alternator output (if there is any?)

All the earth links are correct and none of the wiring has been botched or incorrectly repaired or altered in any way. All I have done is tidied up a few bad connections or re wired/ soldered a tired length that may of been suspect to a future fault.

None of the bulbs are LED and all the Dash light bulbs function exactly as they should.

I think my first recourse is to strip the cable sheaths and check the wiring as per the 907ie wire diagram Who knows maybe there is a burn or brittle cable in there?.

Okay a little progress today as I spent what little time I had getting BMW Cafe ready for her MOT.

The Alternator is happily punching 15V and upwards at !000 rpm and above.
Resistance was as Derek suggested.

So I don't think I'll touch the Alternator cover.

Once again I'll check the visible wiring and then cut away as much of the shrouding along the loom and see if I can find any corroded/burnt wiring.

One thing I haven't replaced is the ignition unit just below the Rectifier - do they ever go tits up ?

Watch this space unless you have any further ideas

Taken delivery of

New Stator
New Rectifier
New Fuel Pump
New Fuel filter
New Fuses
New Electrical connector
New Fuse Box
New Battery

Let the games begin