Ford s max temperature gauge erratic

[Kilfree]

Hi folks. My 2017 s max 2.0 diesel has had it's oil cooler replaced under warranty last February due to oil contamination in the coolant and no hot air from the heater. Since then the temperature gauge has been behaving strangely. It doesn't move from cold sometimes or may go back to cold while actually driving. I got onto the dealer within minutes of collecting the car and they fobbed me off saying that the car would have to be driven for a few days before everything would settle. Then lock down happened so we couldn't do anything but drive the car. After lock down ended the dealer took back the car and replaced the coolant tank and cap which they said wasn't sealing due to the oil contamination. This is frustrating because they were supposed to replace this initially. Anyway it didn't solve the problem. I may have to sort it out myself (I have my own bodyshop) and was thinking of replacing the thermostat assembly (they were supposed to have done this but I am not sure what to believe). I'm afraid that driving the car with it not coming up to the correct temperature will cause other issues in the future. Any help or advice welcome 

Thanks Joseph