Cat just over 1 year old, P0420, 150k miles

January 2019 I had the cat stolen from my 2008 Prius. The insurance company took care of the work as I was on an extended holiday. The car has worked fine until approx October 2019. The orange engine light appeared and when I ran the diagnostic fault code P0420 appeared showing 0.03 reading. When I removed the fuel cap the fault disappeared, but it has come back and this time is not disappearing.
The car has about 150k miles and has been regularly serviced since I bought it in 2011. Fuel economy was very good at 54mpg but now showing 45 since I returned from holiday in March. Oil is not being excessively burnt as far as I can tell.
The CAT is not an original Toyota and has a 2 year guarantee. Is it likely that the CAT is failing? Should I take it back to them? It has passed an MOT and was serviced in November. Very little mileage since, specially with Covid restrictions and another extended holiday. Its been fault free since new and still in great working order except this problem.