Not quite the same thing I know, but the rain appears to have stained my front seat. It should have the 'pureguard' stuff on it (I paid for it!!) so I was wondering if these seats do stain very easily? never heard of rain doing this before.... The dealer is going to have a grilling about this soon!
In answer to your original question you could try a valeting place. Some of the hand wash places near me (Lincoln) will do it for between £20-£50 depending on the quality of the place. Other than that a (good) fabric cleaner might be worth looking into.
Good Luck