MOT: Emissions Fail

Hi there,

My Fiat Punto (2000, 1.2) failed it's MOT on emissions last week. The High Idle reading was: Lambda:1.04
CO 0.65 vol%
Is it unrealistic to try bring the emissions down using a mixture with the fuel or will I need a new converter?

Really not up to speed with my cars so some help would be great!