In his Dictionary of Yorkshire Surnames, George Redmonds notes that the surname has been associated with the Skipton area for about 450 years: the earliest record he gives is from 1576, and there are earlier records from which it is absent.
He refers to the research of Richard McKinley (English Surnames Series, Vol.4 The Surnames of Lancashire), who had found examples in Lancashire in the 13th & 14th centuries. In Redmonds' opinion, McKinley solved the origin in identifying Walter le Demand as Walter Demester, where Demester is a northern English term for a judge (still used in the Isle of Man). Redmonds gives further examples, with variant spellings, from Co. Durham.
Redmonds' book is quite expensive, but very comprehensive; you might find a copy in a public library or a family history centre's research room (probably more likely in Yorkshire). I haven't come across McKinley's work, but presumably similar kinds of library might have a copy (in Lancashire).
