[Austin160]

With Morris 1000s and sticking thermostats, the first you knew about it was a boiling-over radiator as you say, since there wasn't a temperature gauge. Again as you rightly say, this needed careful removal of the radiator cap to minimise the risk of having burns, but all important, you had to wait for the system to cool to a reasonable temperature before adding cold water.

Failure to do this could result in thermal shocking and the potential for cracking heads or blocks.

Back to the original post:- by what I understand of it, would I spend on 'expert opinion' to present to court in the hope of winning recompense? No, I don't think I would.
I think I would do as suggested, and invest my money in a pressure tested secondhand head and go forward with experience from there. If the thermostat was the culprit in this then I would certainly invest in a main dealer replacement part rather than an aftermarket one (which can be less reliable) - but this might have been done anyway.