I eventually managed to find someone who can work on these sorts of electronics. He was able to copy my original body computer data to another one and I fitted the "clone" back into the car. It was with the "foreign" ECU set so obviously I wouldn't be able to start the engine - but I'd be able to see if he'd fixed the "body computer issue". Disappointingly it was still the same; all the same systems not working. He assured me there was no way he could have transferred the fault along with the code (different memories), so we were stumped (not for the first time).
He is continuing his investigation of my original body computer (fitting it to a Punto to see what it does).
It having now become clear that I'm not going to get a solution soon, I started figuring out ways to get stuff working without the body computer - so I could have some sort of civilised existence in the GT. I've got the driver's door window wired via a sprung "dpdt" switch taking a live from the fuse box (piggyback) and an earth in the door frame (hinge bolt) it works. So I figured a similar fix should be do-able for the radio. I have an aftermarket "Pioneer" head unit which I fitted just after Christmas. On which I'd also got the steering wheel controls working.
I figured if I took a permanent live from the fusebox (the switched live is already coming from the cigar lighter so is working anyway) I might be able to get the radio going. Well I did this and it worked. Yippee working radio. Now. I thought, let me look at the heater fan, see if I can get that going in a similar way. To my absolute amazement there was no need - everything: windows, heater, boot lock, mirror controls etc. had come back and is now working normally.
I haven't any idea why this is so, and I can't decide if I should be happy with myself to have got it working, or angry with myself for messing it up in the first place. Because it has now become clear to me that this was a self-inflicted wound.
Somehow, it seems the combination of after market radio plus CTSAR001.2 steering control interface wired up with the switched live from the cigar lighter and permanent live from the original harness makes the body computer go to sleep ( after a couple of months). Replacing the body computer with another one gives a reprieve for a couple of months when that one falls over too. Wiring it with both electrical supplies ( switched and permanent) from outside the canbus system seems to be OK though (although I may just have discovered another way to get a two month reprieve ).
Moral seems to be if you want steering wheel controls on your GT don't use the CTSAR001.x (whatever the vendor tells you) use the CTSAR003.x then you don't need to source an extra switched live and it's all coming via the canbus.