Fault code p0420

One of the Classic family fleet is a 2003 Fabia 1.2 with a valve, AWY engine.

The engine management light came on about 6 weeks ago, with a fault code of P0420 " Catalyst system below threshold bank 1". The engine still ran normally but I found a split in the plastic brake servo vacuum pipe which was quite bad, and moving it caused the engine to misfire. I repaired the pipe, cleared the code and all seemed well until the other day when the light came on again. It has stored the same P0420 fault code. The engine runs fine but I wondered if anyone could shed any light on possible causes ? It has done 128k miles but has up to date service history. Cheers