Engine not getting up to temperature

[GreenV8S]

There's an obvious discrepancy between the engine "never getting up to temperature" and then being "bang on 90 degrees".

You need to find out the actual engine temperature to work out whether you have a cooling system fault or a gauge fault.

[HustleRussell]

Thermostat, the heat of the engine coupled with the lack of coolant circulation with the engine off enables the coolant to reach operating temperature local to the sensor once switched off.

[Tony1963]

Yep. Thermostat.

One way to confirm further:

Drive it for ten minutes or so, then yet it up to 80mph, private airfield obviously. Does it now show 90? If so that’ll be because the engine is working harder. Then pop it into neutral, or dip the clutch. Temperature drops off again? Engine not working hard, lots of cooling air, but stat not closing.
Then, when you get home, leave it idling, stationary, bonnet closed. Back to 90?