Nissan Micra K13 Engine Code: P0420

Nissan Micra K13 engine code advice for upcoming MOT

Engine Code: P0420 Catalyst system efficiency below threshold bank 1

Any idea about easy fix for this? It comes on and then sometimes goes off by itself, otherwise I can clear the code and it will stay off for a period of time.

Probably need to get this sorted for my MOT next month. Any advice appreciated