Stu,

I have had a very similar issue to Dave too, where upon first MOT is was running 0.37% CO, upon second MOT it had shot up to 9.5%. (I realise that you can't diagnose faults over e-mail). Nothing was actually touched apart from a seized rear brake caliper. Its been to a rolling road and they were not entirely sure what was the cause, but said on boost it was running correctly (ie not rich as per idle). They initially suggested the Lamba sensor but after connecting up equipment this was fine, but they did say it appeared to not work up till about 2.5 - 3K then it functioned.

They even checked each sensor and they were all within limits. (lamba sensor was working)

They suggested the chip may be the cause ?? which is in line with what you have said above. I will try that KAT fuse just to make sure.

Is there any other easy suggestions ??

Thanks !