Thanks for the reply mark, sorry mate not sure I understand your point which were you implying was the cause?

Upon checking exhaust temp sensor output I found one of the values was -270.6'c! Checked wiring and stressed the loom in a couple of suspicious looking sections (wires taped together with signs of water ingress) and the temp dropped back to a plausible level, no more fault.

Informed the trader that there was probably a wiring issue to the sensor but since it hasn't put the light on or gone into limp home for 30 miles now (glow plug light would come on after 30 seconds of driving and go into limp mode) he's happy to sell it as is, Surprise Surprise!

Confused as to why there was no fault code relating to temp sensor value/circuit
Maybe someone can enlighten me

Thanks again