bulb fine, no volts and continuity from the tail light to the big relay under the tank that apparently checks for faulty bulbs, but a quick ring to BMW and a quote for this relay well in excess of $200 , so not changing that in a hurry.
So was wondering where I could take a line off another power source as this light can be on at all times the ignition is on?

Is your front parking light working (the one integrated in the head light)?
If it is, this would be the best replacement source to power the tail light. The color of the wire is grey/white and it's going straight to the bulb monitoring unit.

rocket ...depending on age the stop lamp switches on the hand brake and foot brake ....do go faulty ....

usually with some excersize ..and some contact cleaning fluid ...they can be made to work again ....erm sometimes ....another possible location to look is the connectors that join those switches to the main loom ....(which is the best place to test the switches anyhow at the joiners ) get some ...if you havent already ...."deoxit "

the relay you are refering to is know to us all as the "bulb monitoring relay" it looks at stop tail lamp and flasher and parking bulbs i believe

i cant see why this bit of aperatus would affect the operation of the brake lamp ...unless severe corrosion had occured or overloading (short circuit condition happened ) usually the bulb monitor indicator stays on and is a real anoying thing ... without affecting operation ..

the internal operation is across a piece of resistance wire ...to measure the voltage drop of the bulb operation ...the difference signal is then applied to an integrated circuit specifically designed for the operation .....and is now out of production (there is alternatives according to some reference book data sheets i have seen, but require modification to the design of the circuit) ...the week link in this relay is the output circuit transistor to the bulb in the instrument cluster ...which usually dies when some one gets the connector pins to the instrument cluster ...one pin out ....(none of us have ever done that hey ????) this tranni is quite common and could be replaced quite easiliy ...

it would be far easier to import one ...from the great us of a as a second hand working component ...about $30 -50 should get you a goodun ..

but i must say we have seen the terminals within the bulb housing and spade connectors on the mount , needing some cleaning up ...in fact just changing the globe is a recomendation ...i am at pains to explain how low an opinion of recent bulb manufacturer supplies i have seen recently ...with many not lasting a year of conservative service %[email=...@*$^&%$]*$^&%$[/email]* (swear words)

Thanks guys, had another quick look last night and realised the bulb checking relay has no effect on headlights working so a red herring. Had lent my manual to a mate up north, so have downloaded a wiring diagram and will wade through it. Had to use the car for work yesterday, very painful once I hit the suburbs. Need that bike to have a nice fresh warrant of fitness so I can resume biking...

might be worthwile just wiggling fuse 1 rocket ... obviously check it also

reminder to self, when dealing with wiring issues, start with a diagram to follow! thought about the front parking light and of course it was also faulty. hmm, no volts to fuse. Pulled tank off and disconnected headlight switch gear under tank. Also dismantled switch gear for third or forth time
Found I didnt have continuity between fuse #2 and the parking light plug. How do these wires change color part way through the loom?? Assumed the wiring diagram was right and pulled the fuse end out and the pin out of the other plug end and soldered up a fresh bypass line. reassembled and voila pilot light now worked, but still no tail light. replaced the bulb check relay and yay voltage to the rear circuit.
I dont know what happened to the line that failed, its ravelled up in the main loom, but its plugs have been recycled to the new wire and the K has passed its warrant of fitness so all good for another six months of riding.
Thanks for the help. Just having someone to ask is great

yay good result....i guess

in berts interactive diagrams is a circuit of the bulb monitoring circuit, i'm guessing you have some corrosion issues in there somewheres ..