So hypothesis number 11 was that actually the radiator, contrary to everything I have ever said, is actually very efficient. And although the thermostat is rated at 88C, that is fully open and it actually starts to open at 76C. So the guess is that the moment the thermostat cracks open that allows enough water to flow through the radiator to keep the engine cool. So I needed to make the radiator less efficient.
Decided it was time to fall back on the tried and trusted method we used when I was a 'lad' and partially block off the front grille. So 2 roughly cut pieces of cardboard held on with cable ties was the 'Proof of Concept'.
Took her for a 35 min run and towards the end she stabilised at 84C (the small rise at the end was me driving slowly through a village). When I drove back home, scenic 20 min run, I didn't bother attaching the laptop but she warmed up to temperature much quicker and I got the impression she stabilised around 86-88C, which would be logical.
Plan now is to do a much longer trip tomorrow and see what happens.
Meanwhile, did a more permanent solution. Two properly cut pieces of cardboard covered in baking foil. Very Smoooooth :-)