Ian,
I think what you are trying to do is geat. I ask people all the time are we cleaning for health, or to improve appearance. 99% reply appearance. Some care about harmful chemicals, but most don't.
Many years ago I was told that people buy for 1 of 2 reasons. Pleasure or Pain. With carpet cleaning that, IMO, relates to, Pleasure - I want to know my carpets are clean, even when they don't look dirty or Pain- Just get them clean ! If you understand where they are coming from then you can adjust your approach. Those that derive pleasure from clean carpets will possibly enjoy knowing that the clean is ECO friendly as well, the others don't give a damn.
Just a thought
Nigel