Hi Mike, did you have anything further to report?
I have the same car as you with an emissions issue coupled with an ECU error P0420 indicating a catalyst fault (yet to be confirmed). My lambda result was OK on the emissions (1.02), but initially failed on fast idle CO. They did several tests, and it passed fast idle at the very bottom of the specified rev range, but did much worse at the top end. Maybe you need to get a more sympathetic MOT tester with a lighter right foot! These figures should all be printed on the test results.
You haven't said, but I assume you don't have an orange engine lit up on the dash? If you don't have a warning light, then the car itself must think everything's fine, so it seems curious that the actual emissions are poor. Perhaps that does point to a sensor fault? There's a lot of stuff on these forums about what you should see from the lambda sensors if you hook up the car to a laptop.
I should add, I have no expertise in this area, but I'm learning! :-)