As promised an update. Took the car into an Indep. garage for a diagnostic. There were no faults indicated on the OBD. The mechanic searched for a leak on the inlet side but could not find one. He also said that it was unlikely to be an exhaust leak because they made a characteristic sound. Anyhow, the car subsequently passed its MOT and associated emissions test. The Lambda was 1.02 and within the required range of 0.97 to 1.03. The CO readings were still high though (appeared to be in the order of 0.55 to 0.6 %): he 'encouraged' the car to pass, however, by ensuring that the CAT was very (beyond that normally expected) hot. In his opinion I may have unfortunately had a bad CAT (replaced last year at great expense) and the guarantee was only for 12 months. Apologies that I do not have a solution.....albeit I am clear for another 12 months!
Thanks for all your input.