VCDS method of finding oxygen/lambda fault?

Hi folks. My '02 S8 has just gone through the MOT again (spent a freezing weekend in january replacing all 8 front control arms & track rod ends) and just scraped a pass - emissions were only just within tolerances.

My question is this, I have the engine light on, it's been on since I had some sports cats fitted at a place in Sheffield about 6 months ago. VCDS reads the fault as "CAT bank 1 & 2 inefficient" or something along those lines.

The MOT guy said the cats were fine - the emissions suggest it's more likely a dodgy oxygen sensor. He has a diagnosis machine that he reckons can pinpoint which sensor is at fault, but I'd like to try and find out for myself (can't afford his hourly rates ).

Can anyone give me a run-through on how this is done using VCDS, or can you point me to a guide some place?

I know there are supposed to be pre- and post- cat sensors, but other than that I'm ignorant to these things so any advice much appreciated.