Andrew
The cat did eventually cure the problem. That was the last stage of many including inlet manifold removed to fit new plugs and new ignition coils and six new manifold gaskets. Also previous to the inlet off I had fitted both new Lambdas (one at a time and testing after each individual Lambda was fitted. The car had a slight misfire before my P0420 codes started. That in itself can kill the Cat, which it eventually proved had happened.
Word of advice if you decide to go a similar route: always use genuine manifold gaskets (Jaguar, in my engine there were six separate gaskets), always use Denso Lambda`s (Denso being OE equipment) and always purchase a good quality/dearer Catalytic Converter. Cheap East Asian products do not work well on Jaguar engines.
If any other advice needed or questions to ask, please feel free to contact me.
Good Luck with sorting your problem.
Best Wishes and Regards, John