2008/58 2.0 Diesel Fuel temp warning light problem

Car drives fine, and temp gauge sits precisely where it should, with no issues.

Yet on start up, I get an alarm, drawing attention to red temp gauge warning light on screen.

This has happened last few days, and light clears straight away once switch off and re-start.

Not so today, Stayed on regardless, and whilst driving. Drove a mile, switched off, started again, light and alarm gone.

Car serviced yesterday, and as a sidelight bulb was out, they figured this was related. Not so !.

Any ideas ?

Hoping for a faulty sensor at most !

Thanks very much