I have the same problem with the Berlingo 2.0 HDi (guess same engine as yours).
After a cold start then a few miles running the temp gauge will suddenly zoom up from the normal 75oC to over 90oC and the alarm beeb will sound. Within a second or two the alarm cuts out and the temp drops back to 75oC or so. It will then run steady like that for hours --- until MAYBE the next cold start. Does not happen every time, but more and more frequently.
Mine started a few weeks ago, on starting up it would just display the message (when the message is displayed the temp gauge shoots up at the same time) then it was fine. The other day it just started displaying the message every few seconds with the annoying beep everytime.
Had a quick look earlier and pulled the connector off the sensor which then pulled the sensor out so it mustn't have been seated properly (lost coolant again
), do I need to separate the connector from the wires to check if the pins need pinching together DD?