I Think My Catalyst Has Given Up

I've got a MK5 fiesta with 131k on the clock. Ever since i have had the car, the MIL has come on bringing up P0420 (catalyst efficiency below threshold) The frequency of this code coming back resulted in mt buying a code reader from a friend, and I am forever turning it back off! I changed the post-cat lambda for a NTK part which wasn't cheap, and a hundred miles or so later and the codes back. I found pumaspeed sells a MIL eliminator for people who have sports cat/de-cat systems and i thought it was a bit dear for what it is.

I went on the interweb and looked up a DIY "o2 cheater" bought myself a 1Mohm resistor and a 1uf capacitor and modified my original lambda. IT worked well but in the end bought up P0140 saying it was registering no activity from the o2 sensor. One extreme to another!! I don't want to be replacing the cat as it passed emissions fine so there is no real urgency for it to be changed, that plus the cost lol! Though I did hear somewhere.. that if a cat is less then 95% efficient then it will trip the MIL, is this true?

Just wondered if anyone has any advice on how else to try and fool the ecu into thinking its "perfect"?

Cheers

Dave