Following up, the overheating problem was being
caused by the radiator. After I put the flush in, the radiator started
leaking water. So I had to remove it to fix the leak. In the process of
removing it I sheared off the bottom oil cooler nut, ouch!

Once I removed the radiator I found out the reason it was not cooling
properly. The radiator was totally blocked. No air could pass through as
pretty much most of the front had become a gooey mess. It wasn't so much a
question of finding and patching up the leak as replacing the entire core.
However, I needed a new oil cooler too.

So I went to Mercman breakers in Southampton and for £140 got a "new"
radiator and oil cooler. Apparently they are rair. This one has a thicker
core which was a bonus. The main dealer wanted £290+VAT for the radiator
alone. Recoring may not have been possible with this rad as not all Behr
rads can be recored and would have cost £85+VAT.

The car is now running perfectly. Around 80-90 in most conditions.

Thank you all for advice.