Loss of power

I have had both ignition and cat failures and the difference was significant.  The ignition failures created significant misfires when the engine was hot and under load (appeared to run fine when cold/idling), whereas the cat blockages seemed simply to be the engine running fine but with no power.

The cat issue was notable in terms of the volume of gases leaving the tailpipe and the temperature change in the exhaust itself immediately post startup.  You can find this (VERY CAREFULLY) by seeing if there is a part of the exhaust which doesn't heat up evenly shortly after startup.
Stoo

Thank you Stoo, I will check exhaust temperature. Is there any particular part of the exhaust system which will indicate the site of the problem? I should add that the middle section with the two cats, I think that is what they are, was replaced at the MOT.

Another possibility is the MAS, mass airflow sensor, another new concept to me!.. A friend put an OBD monitor on the car this evening and it did not find any error codes but when I revved the car the airflow reading only went from 2 to 3 which is apparently not enough air to feed the engine.
So, 2 new things to check, I might start a new forum question.
Kind regards

On the VVT engines there are 2x cats - one is literally in the manifold down pipe and the other in the first section of exhaust before you get to the boxes.  

These would normally be the least likely sections to be replaced due to rust on the basis of proximity to the engine meaning water stays as vapour and/or water running on the outside of the car during movement is blown towards the back.  Unless you are sure these have been replaced I wouldn't write this off as the root cause.

You may be able to tell from the tailpipe if there is very little exhaust gas coming out...

Stoo