Hi John,
A deep clean of catalyst appears to be using a system of cleaning out the exhaust all the way up to the valves. It appears that there are several experts with the correct fluid to put in and run the exhaust for an hour or two.
That is just what the novice [I am only 87] gets to know. But it worked! My car is a diesel[
When I next see my mechanic I will find out more.
Regards,
Peter.