Thank you very much for your response. The filter is new and the oil is fully synthetic and was changed with the filter 2 months ago. I haven't recently replaced the charcoal canister but I suppose my frustration lies in not understanding the logic of the readings. As you say the readings appear to give a mixed message (I have checked the official printout a number of times!).
If the system was running rich then I would logically work through and replace as many of the associated components as I could prior to the need for the specialist MB equipment. The fact that the Lambda indicates a lean mixture but the CO is high (the Hydrocarbons are well within the limits) is confusing.
Has anyone else any experience of this type of emissions profile?
And in particular, what might underpin the readings?
Any suggestions would be gratefully received