Thankyou for all replies so far

Ok Car has covered 130000 miles

Cat is original as is lambda is, I was under the impression that if either cat or lambda were bad then the CO would also rise ?.

I use about a half ltr of oil every 2000 miles.

Thankyou everyone for your help, this is now just bugging me although the car has a full mot now, I am already thinking of next year :-(