I thought I should give an update about this. I am tentatively considering the problem solved.
About 2 months ago I did some digging around and decided to test what the Fuel Injection ECU was seeing from the sensor. This was with the engine cold (this problem always occurred when the engine was hot). I started by testing the ECT sensor resistance at the ECU connector and, while I don't recall the exact figure now, it indicated a coolant temp of roughly 15 degrees. Seemed about right. Resistance at the sensor pins was about the same. Voltage at the sensor plug between pin 1 and ground was 5.1 volts so that was fine too.
Next I cleaned the earth points on the inlet manifold and the earth strap between the engine and firewall. Whilst they looked clean to the eye the fine wet & dry I used told a different story. A lot of brown crud came off them so I was a little surprised at how dirty they were. I gave them all a shot of contact cleaner and applied a thin film of electrical grease and tightened them down.
As I said that was about 2 months ago and the fault code 123 has not occurred since. Hopefully the simple practice of cleaning the earths has solved the problem. Simple things first.
