Hi I Know this was more than a year ago but i did fix it and the car is still good more than a year later so i thought id post the fix as i should have at the time.

The mode 5 data in obd was part of the answer.
The transition speed of the front sensor rich /lean and lean/rich
was within tolerance but when it was rich it would linger there too long.
I had previously disconnected the maf sensor and noted it made no difference to the lambda sensor on the scope. However later i tried that again but i left it running like that for a while and gradually the lambda switching improved. In much the same way short and long term fuel trim values slowly correct them selfs if you can't reset adaptations. As soon as i plugged the maf back in. It slowly returned to the hanging high thing again.

I would not normal wast my time tiring to clean a maf sensor but as i still was not convinced i gave it a try and it worked.
Now we had a correctly switching lambda.
and after a long test drive no lambda fault codes.
However the Cat was still not working. so we replaced it and once the old one was off although it had looked intact from both ends with a camera in the lambda holes when given a shake it rattled.
Again I'm very wary of after-market cats but one of the better makes did have one listed.
We only replaced the front manifold cat and the vehicle passed it test.

I warned the customer about possible short life
expectancy of after-market cats. They were happy to try as they would not be able to afford the dealer part.

So two faults. possibly one leading to the other
A mixture fault upsetting the lambda but oddly not showing as fuel trim in the normal way that I'm used to. Caused by a dirty air flow meter and resulting in long term cat damage

Hope this helps some one in the future.
Alan