Well,
I had my drivers seat squab replaced under warranty at about 9k, the car is now on about 25k, and guess what, the seat looked knackered again, so, I got onto my dealer, who took some pics and sent them to IM, well surprise surprise, I don't seem to be getting anywhere, so I thopught I'd have a go at giving it a really good clean, just in case, well, whatd'yaknow, the seat now looks almost like new, what I think it was is dust, some of us do dusty work, I know I do, so I reckon the dust that we carry around with us was getting throught the fabric on the seats, and staying in the little cores that make up the seat foam, therefore, it won't come out with a vacuum, but if you persist with a foam type cleaner, you might just get lucky.
I was considering buying a new seat cover, looks like I got £200 to spend. Mmmm.
Ron.