So finally got the dash back in - temporariliy. Plugged the ECU back in and all the switches and wiring. I've been particularly careful about this stage, it's pretty scary cutting in to your ignition loom for the first time!
Consider the following:
Basically you can see the thick battery cables runing from the Battery to the switch and back again to the rest of the car. In the first picture you can see the resistor on the red cable with blue heat shrink - this sends residual current from the alternator back to the battery via the earth (negative), if the cut off switch is used. 27 amp wire joins R2 to S1, the other terminal S1 is then joined to the purple/violet wire and green wire that would have gone to the ignition barrel, as well as S2. The other S2 terminal joins to the black (which becomes black and yellow) wire that also used to go to the ignition barrel, this activates the starter.
I tested all this out by finally firing her up for the first time in a few weeks, with much intrepidation 
. Battery was flat as a pancakce so had the jumper car running ready to start, then connected up the battery, sat down in the Beemer took a breath and turned the big red key! A click, all seemed okay. Switched the ignition on under the red cover, cue some familiar electical whining - so far so good! Pushed the start button and have been grinning ever since!