Fullers Earth - try pureadhesion.co.uk.
You can get Aquamix Poultice from tradesdirect.com.
I've used the Aquamix stuff before, like I say sometimes it works great, but sometimes it does nowt. The trick is to mix it up good and thick and (like smooth peanut butter) and give it 24 hours to work. On occasions it makes it look worse, in which case you know it's working and you have to keep going until it gets it all out. But I'd be trying a strong cleaner with the poultice as an abrasive first, with a good 15 minutes dwell time. Using a poultice overnight is generally a bit of a last resort.
I'm sure you know this, but the best advice with any stone is to test a small area first. And manage your customers expectations downwards, you may not be able to shift the marks anyway.
Incidentally, if it is limestone (or trav); I just emailed an old mate in the stone trade and he said to give ammonia and rubbing alcohol a go if that doesn't work. You althernate them apparently, letting them dwell on a terry towel pressed onto the stone. Never tried it myself though.
Good luck.
