Update a week on. So far so good. The problem has not returned....
The specialist had the car up on the ramps and said the cats were fine as far as visually inspecting them and tapping them permit. They ran the tests as I did but the fault did not returned and the suspect injector was showing the correct impedance.
The faulty lambda is still showing and is on the drivers side, sitting directly under the air intake hose joining the throttle body and airbox, so I am looking to replace it myself.
Has anyone used Bosch lambda's on their A8/S8? I suspect they are probably the same as OEM, which are £133...
I have found several on Eurocarparts, with the Bosch unit (see link) looking like it's correct @ £68.
http://www.eurocarparts.com/ecp/c/Au...sor/?710442005
Thanks