Peugeot 207 "depollution system faulty" with awful smell??

Hi, I have a peugeot 207 cc, 57 plate and just last week it stared really smelling of fumes when I put the fan on...Its now saying depollution system faulty and still smelling. Has anyone had this problem before? I've been looking through here and i'm starting to worry that its going to cost me £100's to fix :( Any advice would be greatfully received