Peugeot 307 SW - Anti pollution fault - redcondios

Hi. My first visit here and hoping for someone to help me please!!

I have a '54 Peugeot SW petrol 2.0 and the antipollution fault message is showing and the car is juddering. Having scanned through various forums I am confused and need some clarity!

One site said it is something to do with little magnets on the filler cap - I have looked and there aren't any magnets on my filler cap - is that why I have a fault? Another site said it was to do with the cerium additive...

Basically I need to know what it means and whether anyone knows a decent (and honest) mechanic in South Oxon who can fix it without having to go to a Pug dealership.