307 antipollution fault

My 02 307 2.0 petrol hatchback, occasionally posted an antipollution
fault on the LCD display, and an amber engine warning light on the
console. I took it to be serviced by Continental Cars in Auckland, NZ,
at least 6 times, and each time, five days later, the message
reappeared. Quickly running out of money, I cut my ties with the Peugeot
agent and used an ultra-premium fuel (Mobil Cisco 8000) for 6 months and
the lamp hasn't come back on since ever the change. The service manager
at the Peugeot agency, that I mentioned, phoned to say that they had
recently discovered that by removing parts from the exhaust system and
running the car engine temporarily without the back-pressure from the
antipollution gadgets - they can flush any carbon buildup from the head
without having to remove it - and would I like to spend another small
fortune? No I wouldn't. The Peugeot 307 engine is OK, they thoroughly
tested and tweaked it, there is nothing wrong with the car, it is smooth
and quiet, economy is excellent, engine temperatures are well within
norms and consistent since new, and all I have to live with is the
prospect of having to drive it with a tank full of the most expensive
petrol that I can buy. Can anyone with experience with this problem,
verify that the Peugeot service agency is a liar and a thief and can
they recommend a way to ensure that the antipollution fault and lamp
issue never reappears? Post it here, don't email me.