Testing Thermostat on a Disco 2

HELP IM STUCK
After having both head gaskets done, fitting a new thermostat and flushing and bleeding the cooling system twice I still can’t get my heater to blow hot unless I rev the engine!! What’s worse its starting to get hot!!
For story so far please see This thread

Today I fitted a new water pump feeling sure that it would cure the problem as it was the only component left in the cooling system that hadn’t been replaced by me in the last few months. But it has made no difference.

I’m now thinking the new thermostat I fitted at the weekend is faulty? I’m sure there is no blockage anywhere, but if I left the engine get hot the top hose gets hot but the bottom hose from the radiator to the rad stays stone cold, and the 2 hoses from the top of the stat are get hot.

Also since fitting the new stat the tem gauge has started to go past halfway this is something that has never happened before not even before the heads where done . I drove it a short distance today after changing the water pump and the heater was still cold but the water in the header tank was that hot it started to escape through the overflow?

Is there a way to test the sealed unit Stat of the car?

Cheers

Presume you have replaced/flushed radiator and done a complete cooling system flush?
The heater matrix can silt up & only become hot when you rev the engine & increase the flow/pressure.
How quickly does the engine get up to tempriture? you should feel the top hose and rad become warm when the stat opens.
Also has the engine to your knowledge always had a 50/50 water antifreeze mix, if someone has run it with plain water in the past corrosion of the alloys will cause a silt build up poor circulation & overheating.