I came across this post on another BBS http://www.cruisersforum.com/forums/f118/anchoring-in-mud-53646-8.html#post650460 posted by the Fortress salesman. (Craig Smith's counterpart).

A good tip on setting the anchor when the bottom is soft mud.

(BTW, apologies if this starts a huge anchor thread, but this was something I hadn't considered before and think it might be of general interest).