Hi Ianmc,
I purchased an OEM Cat. Reading online many people promoted this as the cheaper ones are not up to the quality required. You can also replace the Lambda sensor, but I was only going to do this if the car failed its MOT. I paid £230 for my OEM one and the product code is B90414H the ‘H’ is the important part of the code as it deems it OEM quality. The company that produce these cats don’t sell direct to individuals, so you have to go to a third party seller which I had no problems with.
Hope that helps!