CAT problems UK

My exhaust fell off my Toyota Prius 2004 Gen 2.

I took it to Toyota to get a replacement back box (around £150) which was not so bad. However to my amazement, the exhaust was damaged near the cat, meaning I had to get a whole new downpipe. Toyota quoted me for the official CAT at £1600 or a third party CAT at £500.

I fitted a third party CAT via Toyota. After 3 to 6 months of driving I started getting the engine management light come on with the P0420 code.
Testing the CAT Toyota found the readings are fine and within the threshold.
Toyota subsequently fitted a new heated oxygen sensor, and after a week the engine management light is back on.

They have now replaced the CAT under parts warranty and after a week of driving, the engine management light is back on.

The only course of action is a £1600 CAT and I can't afford that.

Any advice appreciated,
Thanks
Manner

 

Yes I am burning a lot of engine oil. I have since had a replacement aftermarket CAT from Toyota, after 1000 miles I have again received the engine management light and the code P0420. Now Toyota say it requires a genuine CAT but I must fork out £1800 for this CAT.
Has anyone had any success with an aftermarket CAT?
Could it be worthwhile replacing the air/fuel ration sensor?

At the moment I am going to get the CAT replaced again for another aftermarket CAT of the same quality and make. However, I think I will just end up at square one.