I worked with 2 friends of mine who were professional carpet cleaners. Top of their game as they could clean hand made carpets (one of them had a relative who owned Signature Carpets who make hand made carpets – we are talking £100k+ for a ground floor for a big house – they were the only people they would recommend to clean them)
They said a lot of these guys who quote really cheap prices for cleaning your carpets used nothing more than a Rug Doctor (which are ace, I worked in a place that hired them and they always got good reports) and quite often left the carpets very wet.
They had a transit van which cost £50k to fit out at they had a 15hp diesel engine which powered their cleaning equipment. They used much harsher chemicals which made things much cleaner (but he used to time it to the minute how long to keep them on the carpet before sucking them up) and obviously had huge sucking powers which he said was the key, the sucking out of the dirt was the most important bit, not just setting it free.
They recommended cleaning your carpets once a year from new. A lot of people say “if you clean them they get dirtier faster” which is total crap. The only thing which should make you change your carpet is if you want a change in colour or it wears out. Amazing that most people spend thousands on their carpets but never clean them properly.
They have moved to the Lake District now – they do do private homes and not as expensive as you might think but the service is amazing. I can find their details if you want.
Rug Doctor +1 as well 😉