Gavin,
Yes our livng room is near enough 35 sq m, but I doubt I charge as much as you to afford it, the North / South divide is apparent in lots of differing aspects, house prices being one and cleaning services obviously being another.
I would probably charge about £55- £60 to clean my own living room carpet and I would be happy to get that price for the job. When I do quote it is generally on how long I expect it to take rather than the actual measurements.
Within this region I am not considered cheap, like you I promote a superior service to others and rely on recommendations and reputation for getting a lot of work, often without having to quote for the job prior. I like to think people know they will get a professional job and will be willing to pay a fair price for it.
There is a cc in this area, no names, who advertises £22 for any carpet, before Christmas he was doing it for £18!!!. He appears to be just as busy as me and I have not heard any negative comments about his standard of work.
There are without doubt areas in the country where price is no object, people just want it done, I would love to be in such an area. But there are probably as many areas, if not more where you need to be price competitive to get the work, or you sink very quickly.