HM22C001 Fridge/Freezer always running (too cold)

I have a Hoover HM22C001 fridge freezer that has recently begun to freeze everything.
The compressor is always running, and changing the thermostat setting does nothing to stop it.
So I thought… EASY! It’s the thermostat.
I bought a new one… fitted it without any problem and… the damn thing is still faulty!
So the symptoms are:
Compressor is always on, unless the thermostat is turned to 0 (OFF).
Turning the thermostat back on does not result in the compressor immediately starting. It waits a minute or so, and then switches on.
After that, the compressor will not switch off unless I set the thermostat back to 0 (OFF).
Looking at the thing, I can’t see what else could cause this!
HELP!