might just be me, when car fully warm and have heat turned to max and blowers on max doesn't feel hot, warm but no more, am sure used to be much hotter. Car temp gauge is fine.
New thermostat ?

Maybe related to your low coolant I would guess

ok

Thermostat.

Does the temperature needle sit in the middle of the gauge when the engine is warm or does it sit just below?

Go into the hidden dash menu (hold the indicator stalk button when turning the engine on, when it says "test" on the display, release the button.) Press the button to scroll through the options until "coolant temperature" is on the screen. Go for a drive. It should get up to and hover between 88-92 degrees C.

If it doesn't get there, the thermostat is stuck open, meaning that coolant is always flowing through the radiator and not getting hot - which will also cause the heaters to be cooler than you'd expect as well.

Thermostat is a bit of a nightmare to change only because of access - a remote hose clip tool makes this light work and cuts the job time and injuries right down. Part is about £30 IIRC.

If it's not the thermostat and you're also losing coolant it could be the water pump. This is a cheap and easy fix if it is that.

FWIW, I replaced the water pump in my car at ~60k miles, and the thermostat got stuck open at ~75k miles.

temp needle always sits just below middle from when I bought car, I've never seen it higher or lower.
Don't think I have a leak however have topped it up tonight to just above minimum, that should allow me to see if it drops again.

Below middle definitely points towards thermostat stuck open. When the car is up to temperature the temperature needle should be sitting precisely in the middle. Try the gauge test to see what the temperature actually is.

Here's where mine was sitting after a half hour drive:


It should be a constant 88-92 degrees.

mine definitely below yours, I did try to access the menu this morning but seemed to get into settings Language, controlling door locking and other stuff couldn't find my way to temp.

Think most STs have had a thermostat needing doing, seems very common.

this is where my temp is

Yep, open thermostat, almost definitely. That's how mine sat. The photo I posted earlier @ 77C was the hottest it ever got, and that was after the engine was switched off and air wasn't flowing through the rad. When driving it would sit at 67C ish, which given the nature of the programming of the temperature gauge is about where yours is sitting.

car due for service in May so will hang on unless develops worse

One thing you'll find is when you get it done and the car runs at the temperature it's supposed to - it'll run so much better, plus you'll get a bit of extra MPG, if that's your thing.

popped into Ford today, they will put a new thermostat in for me when in for it's joint service/mot next month.

where is the thermostat, someone said the mountune inlet will have to come off !

No, it shouldn't do. It's underneath the power steering pump/reservoir. A real pain in the arse to get to, but doable without removing inlets etc.