Just a bit of advice, probably not so important either as our cars don't have cats but for those that do...
Do not buy a cat from cats2go.co.uk - I bought one last year for my girlfriends Skoda Felicia after the original broke a weld. First off it didn't fit properly, despite being the 'correct' part number for the car. So it required some interesting bodgery and cutting to get it all to work. Anyway, it failed it's MOT on slightly high CO emissions last week and I was advised it was the cat - to my surprise!!
First test with 1yr old cats2go cat: Fast idle CO: 0.4% (Max 0.3%)
Put the car in for a retest with free original cat fitted from a 10yr old Felicia: Fast idle CO: 0.005%
The original system had no audiable/visible leaks - so it can only be the cat as that's the only thing that's change! Oh well, another year for free...