Removing water marks from dark grey upholstery?

[pimpin gimp]

I'm looking to sell the A6 at some point in the near future and it's pretty tidy except for the front passenger seat which has a lot of water staining on. What's the best way to get rid of it? I've had bad experiences with upholstery cleaner (Halfords) but that was on light fabric, will it fare better on dark charcoal (I think) fabric?

Should I get a detailer or valeter to do it, or should I get a rug doctor? Or should I just buy some better upholstery cleaner?

Cheers in advance.