I too believed it's the testers machine when the MOT failed. But since then 2 other garages I've taken it too have confirmed the same levels of CO, consistent with MOT readings.

So yes I asked the same question to the technicians as to why the post-CAT lambda sensor wasn't picking this high percentage and they didn't have a clue ?

Where about are you based, in case I can potentially drive it to yourself and hope for an answer.