I have made inquiries and looked under the bonnet. Plus Four CXs without AC have one water radiator and this was attached to the swirl pot when the latter was fitted. I am told that this original radiator was a 2 core radiator and the new one now fitted is a 3 core radiator (presumably better able to cater for any residual thermal or other shock).
I suggest that all who have had a swirl pot fitted check for water leaks regularly and also check that any new radiator is a 3 core radiator or at least and improvement on the one fitted when the car left the factory.
I recognise that an experienced motor engineer/mechanic may wish to comment on the above. In any event it seems unlikely MMC will ever explain in clear terms why an improved radiator was not fitted when the swirl pot was fitted given the apparent cumulative effect of thermal or other "shock" over the period of use of the vehicle.
I note that owners of pre CX models have had radiators replaced but they appear to have been isolated incidents.
The additional cores would have little effect on thermal shock resistance as that's (allegedly) been caused by the thermostat opening suddenly as the system sees high load and sudden temp rise
My experience of replacing radiators with uprated versions with additional cores (& hence fins) only really does a couple of things, increases coolant capacity in the system and exposes more area to the airflow to increase cooling, eg, when fitting a race engine into a car with very limited space to increase the width & height of radiator
