Roljoe, in addition to the previously noted law of gravity, note the large red outlined mass in the MET ash forecast (latest at
http://www.metoffice.gov.uk/aviation...1271526609.png ) denoting surface to FL200. Of note in the latest is the remark that "showing thinner layers didn't provide any significant additional information."
This seems to imply that that big mass of fine ash is forecast to descend slowly through all FL (that gravity thing again). Of interest are the more recent sampling efforts to gain more information on actual dispersal, especially in primary flight paths.