2002 Vauxhall Corsa emission light on dash board keeps appearing

I wonder whether anybody can get me some advice; about two weeks ago the emission light came on the dashboard; I pulled over to have a look what the problem could be and it said carbon emissions on the catalytic convertor contact local vauxhall dealership; I took the car to the local Kwik fit garage to get them to do a diagnostic report only for them to charge £30 to tell me that one of the cylinders on the cat had misfired and he could charge me another £30 to reset the dashboard and that it could just be an act of god; he also informed me that if it was the catalytic convertor you would hear it when you revved the engine. I didn't pay for him to reset the light I thought I would think about it; I went out that evening revved the car and the light went off so I thought maybe it was damp as I had been driving through a ford a couple of times that week.
Today (21.12.09)the light has come back on and keeps flashing; the car is a little juddery and jumps a little, could this be that it it just cold? as it has been in the minus temperatures (or am I being a little blonde and naive) :0s

I really want to avoid forking out for yet another diagnostic report so close to christmas,however I obviously do not want to cause any damage to the car.

Anybodys help would be greatly appreciated.