JJ
I've been here - well almost. My cat (only one - dealer cost around 1400GBP) I thought was on the way out due to higher than legal limit test result of 0.9%, however after some serious heating the Cat starts running far more efficiently bringing it back under the limit of 0.3% (for my age of car).
I looked into after-market and found a couple of reputable cats for around 500GBP but in the end decided on an original. Mine is 13 years old so that's pretty good value for money and you have to way up the cost against how long you wish to keep the car. For me it was at least 5 more years and with the thought of after-market cats typically failing within this time!
Here's a post discussing lots of things regarding emissions
O2 sensor test and inverse duty cycle
and
UK emission limits