Rover 416 failed MOT due to high CO emission

Hi group,

My Rover 416i (1996 P-reg) just failed its MOT due to high CO during the
emissions test. When I had it serviced a couple of months ago, the mechanic
did tell me that because I mostly use the car for short trips around town
and to/from work (only a 15 min trip) that was the reason for the high CO
output. I did try taking the car for a run on a 60mph road for about 10-20
mins before the test but obviously that didn't help. So, my question is,
should I take it out on a longer run before it's re-test or should I get it
checked for a possible problem? The car's not showing any problem signs and
did pass all the other MOT check items. For reference here are the figures
from the emission tests:

Natural idle:
Engine speed:    877 rpm (pass)
CO level:           0.27% (pass)

Fast idle:
Engine speed:    2700 rpm (pass)
CO level:           0.63% (fail - max allowed 0.3 %)
HC level:            53 ppm (pass - well below max of 200 ppm)
Lambda:            1.01 (pass - almost perfect, should be between 0.95 -
1.09)

Second fast idle:
Engine speed:    2777 rpm (pass)
CO level:            0.68% (fail)
HC level:            85 ppm (pass)
Lambda:            0.99 (pass - again almost perfect)

I look forward to hearing your views/opinions and thanks in advance for any
info you can provide.

Cheers,
Salim.