This film is based on a book called
Pobby & Dingan by the English author
Ben Rice. You could try to get a copy of the book to see if the name is explained in it, or you could try to contact the author through his agent to ask why he chose the name:
http://www.contemporarywriters.com/authors/?p=auth03B5O021512634968
My only guess is that it might be from the English surname Ashmole, as in the founder of the famous Ashmolean Museum.