I got it sorted. Today at lunch time i had a play. Rigged up a 500r pot to the brown/green wire from the instrument cluster and put the other end to ground. To my surprise it worked! I could adjust the gauge up and down over the range 20-220 ohms. As the resistance increased , the voltage on the wire increased. 2v = 1/2 tank , 3v = 3/4 etc. Again though i could "fill" much faster then "drain". There is some form of damping in the dash computer.
I still have to play about getting it calibrated but as this little video shows it works a charm! Well done Chris
__________________
Now, Cole, when you shift the gear and that little needle on the ammeter goes into the red and reads 2000 Amps, that's bad.
www.evbmw.com