A year ago my 2009 1.4 GP went for its 2nd MoT and the Fast Idle Test CO reading was very low at .005%
A year later the same test gave a result of 0.020% (still well within the permissible limit of 0.200% but a considerable change in just 12 months and just 3000+ miles.)
Can anybody hazard why such a difference occurs? Is it something that should worry me?

It will vary as a result of lots of things, engine temperature for one. I bet if you re-tested it again ten minutes later you'd get another totally different reading.v

Thanks James; that makes sense as he changed the gearbox oil for me before MoT test

I think James is right, mine was 0.005% (?) This year but higher last year at 0.020%