I've done a bit more reading on the subject of 'star' ground systems and I am even more convinced than I was before about using this type of setup. I thought the 'forest of tabs' bus was connected to the firewall but I was wrong and it should be isolated and then wired to the pass through bolt with a single wire. The pass through bolt is bolted to the firewall to ensure it makes contact there. I was also thinking of grounding the battery to the airframe using a tab near the battery but this, apparently, would be another ground return and potentially a ground loop source.....so the only wire going to the battery negative will be the one from the pass through bolt.
I'm still waiting and dreaming and obviously just have 'book learning' on this subject but it certainly seems the way to go with modern avionics.
This is my final basic system design (well, it's not mine at all, just re-drawn )