3.5 carb where is the thermostat located

Hey i have a 3.5 discovery engine was originaly efi but put on SD1 carbs now the heater guage flies up and down all the time but i have had the collant drained refilled and bled at the garage and still does it so i am going to change the thermostat, does anyone know where to locate the thermostat? And how to test it
Cheers Steve

In the housing on inlet, test in pot of water

As per Dr instructions
I would replace it even if it is opening. For the low price of a new one

Waxstat is £2.80 I think, so not much and they're good, gasket is 20p or something around that too I think

Yh got one today but not a gasket so need one of them