1.0 Petrol Life (sorry)

I fitted a new type approved cat (BM brand) to my wifes car in Nov 2018 as the original had split at the weld. The car has been running fine and dandy, used daily as a commute to and from the local hospital. This morning it went in for its MOT renewal and it failed on emissions. So I also tried adding some emissions fuel additive, drove it 50+ miles on the A75 and the MOT station 're checked the emission levels and their computer still failed the vehicle. The tester allowed me to personally view the 're test and the on screen results. It passed okay on the Fast Idle Test but failed on the Natural Idle Test. Reading 1.95. Should read less than 0.30

Is there anything I can do other than replace the Cat again

Cheers