LDV Pilot Van 1.9 engine

Recently bought a LDV van 1.9 engine year 2000.

It has just started showing the warning temperature light on the dashboard.
Not bright it is just glowing slightly.

Curiously the temperature gauge is below 1 quarter.

So the cylinder head seems to be getting hot but the temperature
guage is still showing fairly cool.

I removed the radiator lower hose and flushed the system, lots of rusty water so looks to have been missing antifreeze. Did this two or three times.

But still the problem. Next thermostat.
Could it be just a faulty thermostat?
Suggestions please.

The light may also double up as a coolant level warning, so would check that first.

Replacing the thermostat seems to have cured the problem.