They ask if we want to help "save the planet", and enclose a forest's worth of paper in posh bible size brochure explaining tests etc.
You let them take your supposedly fuel efficient car and they see if manufacturers claims are still valid after a few years of driving. If your vehicle fails any tests they tell you but it does NOT affect MOT or Tax.
As I have a 2nd car I can use I am going to do this. Anyone else been invited??