The one bit I was somewhat apprehensive about was the wiring. Given that the donor vehicle was half full of junk so I had poor access, I'd had to blindly snip a few wires under the dashboard, so wasn't really looking forward to tracing out where they were all going.
With the wiring diagrams in front of me however, it quickly became apparent that things were a lot simpler than they initially looked. In fact, this whole lot turned out to be absolutely nothing to do with the EFi system.
The next discovery was that the diagrams had pin numbers for each of the connections to the ECU sockets...which even more conveniently were also stamped onto the back of the plugs. This made it a really trivial task of ten minutes to trace out what the four remaining wires were using the continuity checker on my multimeter.
Three of them were heading to a socket heading towards the instrument panel, and turned out to be the switched 12V supply which feeds the coil in the relay which supplies the whole system, the 0-5V signal wire for the tachometer (which I won't be using), and the 12V wire for the check engine light. EDIT: Just had a better look, the ECU actually provides the *ground* for the CEL not the supply, that comes from the factory instrument panel supply. That'll teach me to read the diagrams properly...
So I'm left with a completely labelled EFi wiring loom ready to go into the car.
One thing I do need to pick up is a lambda sensor spanner so I've got some hope of getting that out of the downpipe without a massive battle.
Was astonished how little electronic detective work I ended up having to do here...it's astonishing how a bit of commonsense at the design stage and a good diagram can make your life easier.
I'll need to post up the EFi wiring diagrams for at the least Jim's benefit - they're quite possibly the easiest to follow scribbles of their kind I've come across. Citroen could learn a thing or two from them!
I can see this happening a lot sooner than later Zel 
