Well, we tested out the wiring loom and everything checked out A-OK
We then wen't through all the earths, which also initially checked out OK, then when I had the engine running and went to operate the throttle by hand I had to move the pipe from the oil breather/catch tank to the Aux. Air valve and the engine cut out.
Went to start the car again and she didn't want to start.
Got out wiggled the breather hose, wen't to start up and bingo off she wen't. So wiggled the breather pipe and the engine died again.
This time I left the ignition on and wiggled the breather pipe and I could hear the injectors powering up then down and then up again and so on. Checked the earth connections and they were good, so took the earths off and pulled back the sleeves that protect the three injector earth wires. I found that there was damage to the earth wires just inside the sleeve to one set of earths which was not visible when the cable & sleeve were in position. The torque/vibration when the engine was running was cutting the connection to two of the wires - and then the engine would cut off presumably because the ECU detected that some of the injectors had powered down.
Cut the damaged section, put new connectors on and fired her up, revs all the way to 6500+, roars like a lunatic and I'm off out now to drive her properly.
I need to take this opportunity to thank everyone who provided input and made suggestions of things to test & try out. I was just getting to the point where I was looking for people to test the ECU and also maybe try another ECU and then it gets sorted. I don't like electrics at the best of times but it's all part of the fun! So, I am very grateful and full of appreciation for all your assistance.