Mark+4, I have already experienced the same issue. Rog and Richard are absolutely right and they have explained it very well. In my case it was the same. The more powerful new aluminium radiator meant that my engine was no longer cooled on the motorway at high speed and it was without very good cooling performance.... I have not yet done anything new in terms of a different thermostat that opens earlier because everything works so well as it is. I have made the "better" radiator slightly less powerful so that it works at the same level as the original plastic radiator. My problem was never a lack of cooling from the original radiator, just that it takes a while for it to burst.
Here in the link below you can see my OBD measurements with strong cooling performance above and counter-intuitive much too high cooling water temperature. Below the cooler and right coolant with reduced radiator output, which allows the thermostat to open.
https://www.talkmorgan.com/ubbthrea...the-cooling-water-temperature#Post713083
