Catalytic Convertor Fault/Anti Pollution etc?

Hi, got a 307 xsi, had a cold start to the morning here, engine spluttered and took a few attempts to start it. When finally running I got the "Catalytic Convertor" & "Anti Pollution" warning message. Followed some other advice from the forum and drove hard and fast for 30 mins. The alarm was going off all the way until switching engine off. Once restarted I just have a single message "Catalytic Convertor Fault" beeps only once and then just the little fault symbol between the speedo and the rev counter. Is this really an issue? Or has anyone experienced something similar? Will try battery unplugging meantime.

James, Oxford
December 2011