DIY Catalyst/O2 Sensor Replacement

[Bisonhead]

I have code 16814 - Catalyst System; Bank 2 p0430 - 002 - Efficiency Below Threshold - MIL ON

Been like this since the 2nd day I had the car (I think the dealer reset it before I tested it and then again before I drove it away). It scraped past the emissions test in the last MOT and I am looking to get it fixed as the light wont go out and stay out.

I am assuming that there is excess soot in the cat. I have been advised this is the case by a reputable indie in Burwell, Cambs and black smoke/soot coming out the exhaust when revved seems to confirm this.
I have tried Cat cleaner/additive and keeping revs at around 5-6k on the motorway for a few miles but this hasnt cleared out the soot in the catalyst and the emission lamp remains on.

If this is indeed a cat replacement, has anyone attempted it themselves? I was quoted £2000 by another indie in Portsmouth which I simply cannot afford at the moment. If I knew it is a DIY case I would happily go for it on my own.

Any experience or similar tales would be greatly appreciated! As would any mechanical help in the Hampshire/Surrey region