Today I drained/flushed the cooling system, fitted the new (BearMach is near) sensor and re-filled the system.
Ran the engine in idle, nothing more.
We'll have to see if it makes a difference.
At least I understand my 3.5 V8 EFI a bit better now!
Thanks.
ps. the plastic screw plug on top of the radiator is rubbish. Need no expanation.