NY wrote :
Maybe the needle is telling porkies... Hopefully it *would* rise into the
danger zone if the coolant really *did* get hot - eg if a hose burst -
otherwise it's not a lot of use in indicating a fault.

No doubt, but it wouldn't give much warning, just a sudden movement up
to max after the engine has seized. I doubt it would be much help even
if the loss was slow.

That is why they did a little gadget which plugs into the OBD,
indicates true temperature and sounds an alarm over a preset
temperature.