Hi Lee,
I have finally found the time to do the test from cold as you suggested and I am happy to conform that the oil temp reading acts much the same as yours did and coincides with the ESP light illumination.
As you can see in the attached images the started from cold and all seems fine. After a few minutes of running the temperature climbs and sits at a sensible level.
After approx 10 minutes of running the temperature rises to 'High Temp' before illuminating the ESP light and suddenly dropping to the negative number as you suggested.
I fast idled the engine from then and the ESP light went out and the temperature rose to a reasonable value again.
Please can you provide me details on the part(s) you replaced to rectify this.
Thank you for all of your help thus far