Sorry for the 2nd (in probably a long line) question, but I've extended out the ZX12 loom and now I'm just adding in the extra bits and splitting stuff where neccesary etc.

My basic question is - the main black/yellow goes to ground, should I create another ground to the chassis somewhere at the back of the car or does it just need one overall??

You could get away with just one overall..

But my view on it is that its better to have three or four earth points allong the loom to make sure that you have VERY GOOD earth connection... as in previous projects it has always been the earth that has given me the most hassle!

As Tegwin says 3 or 4 strategically placed earth points on the chasis is best. It should help to simplify the loom and avoid long earth cable runs.