Any ideas why this might be? CTS and thermostat have both recently been replaced with no issues until today.

CTS was a genuine one and the thermostat is a Gates one.

Might disconnect the radiator fan if the connector is easy to get to as I don't do much city driving so unlikely the temp gauge will setting the other way.

Shouldnt make a difference with climate on.  Should get to 90 and stay there at all times.

 

Sounds like thermostat.

Update on this if anyone else searches...

 

I came to the conclusion that the new thermostat must have got stuck open, it only did it for a couple of days and is now working as it should (warms up quickly then sits at 90 on the gauge and doesn't budge). If it happens again I'll look at getting it replaced again.

 

The fans being on was because the A/C was on - switching the A/C off switches the fans off. Owned the car for nearly 4 years and never realised this!