Hi I have a 307 sw and that had an anti pollution warning plus the engine management light on. I cleared using a cheap code reader, it told me it was an exhaust problem, subsequently the light came on again.
Personally I'm going to leave it, I have 1 206 2 307 s and an 806, a bit of a collector, they all from time to time surprise me with lights and warnings, my experience has shown if it doesn't affect the running of the car and passes an mot it's generally a sensor issue.
If you want to get it fixed for you piece of mind I'd suggest go to a reputable peugeot garage.
As for the place you bought it,it seems they have misled you a little, may be worth pushing them, from what you wrote it was only a couple of pounds difference between with and without guarantee, if I'd spent that much on a car I'd want 12 months free motoring not less than 3 months.
Good luck
Hope this helps.
Dave