Nick,
The photos are great, thanks for taking the time to post them. I will probably use the same earthing point as you, but will try and run all my other cables down the middle of the car under the center console as I have a switch mounted there to control the IGN signal to the PC power supply.
My PC is mounted on a black felt covered MDF board cut to the same size as the original. It has a slot near the back for the cables. All being well, next weekend, I will get all of the bits installed...
Cheers & Beers
Stewart