Martin, thank you for posting your findings in regards to the thermostat.
I would suggest that you indeed fit a Kaiser metal thermostat housing, this component takes away all of the worry and stress of a poorly designed original component leaking at random.
There will be people who erroneously insist the problem is caused by abrasion of particular components leading to leakage, but you have demonstrated perfectly in the case of your own car, that this simply is not correct.
Kaiser can be contacted
HERE
Brian