I had a moment earlier and checked the plug and connections to the temp sensor. It looked OK.
I put my meter on the plug with it disconnected and measured 5V with ignition ON. I wriggled the wires along the loom to see if I would drop voltage/connection but nope it was stable at 5V.
Does anyone know the terminals numbers at the ECU? I would like to check for any corrosion and check the resistance of the cables while I'm at it.
If that all checks out I'll replace the temp sensor and hopefully that fixes the issue.