David Aiketgate

I would suggest that you go back to the basics on emissions.
That is 02 sensor and the catalytic converter.

Your reading for 02 sensor voltage does not look right. Do you have a graph of the voltage available? It should look like this.

Is it the original cat or has it been replaced? If so, was it replaced with a cheap one? Cheap cats are notorious for failing after a short period. (They usually manage to pass one MOT but fail the next due to minimal amount of reactive metals.)