Weird temp gauge problem

Temp gauge has started playing funny buggers,on start up it takes a while to rise to the quarter mark then after about 7 miles it reaches the half mark where it normally sits. This is normal business as I have read on here they are slow to reach normal temperature especially in the now cold mornings.Here is the weird part over the past few days it rises to quarter mark as normal however after 7 miles it drops to below zero and the red warning light comes on.i stop the car restart it gauge goes to quarter then a mile later goes to the half mark and stays there. It has happened every morning for the past 4 days. Any ideas what it could be ? Mate says sticking thermostat.

If a Td5 check the coolent level.

It is a TD5 and coolant level seems ok , it was a bit low tonight however engin was hot I,ll go re- check it now. Heater works fine both hot and cold air no problem.

Just checked the coolant level and it was a bit low ,it took about a 2 pint to top it up. I'll see if it happens again tomorrow.Any ideas why the low coolant level would cause the temp gauge to play up ,remember the tank was not empty just low.

sender wont read properly unless its submerged in coolant ,it would read low if it was in air

Ok thanks ill see how it goes tomorrow .

Ok driving home tonight same thing happened started up gauge raised to quarter, after around 4 miles sat there for another 3 then dropped to below zero and red light came on .stopped the car turned off engine turned back on straight away it then raised to the half mark with no red light straight away and stayed there until I got home which is about another 14 miles. I have ordered a new thermostat and coolant temp sender. Any ideas what's wrong with it ?

Maybe.... an intermittant sensor connection and when the BCU see the lack of a connection the lamp illuminates, switching off/on the ignition resets the BCU till the next time.

Yeah kinda what I was thinking more of a sensor issue. Will find out after I replace the coolant sensor and thermostat . Not too expensive to replace and a good starting point.