Original signed pastel illustration-[Cash in the attic job]

85cm X 65cm This illustration was done by the author/illustrator David Rook, who wrote the Ballad of the Belstone Fox which was made in to a film, The Belstone Fox,this lead to the Disney production The Fox and the Hound.
Other books by him,one, Born Wild Born Free, about horses, also made into a film.
He was an illustrator for other authors books as well.
I have trolled the internet trying to find some of his work to establish some sort of value for this pastel and the only positive thing I have found is prints of film posters illustrated by him that sell for £45.
This is the genuine article as it was given to my ex-sister in law by him, when she worked for the publisher who printed the first edition of the book.

Where will I get an honest appraisal of the value of this picture? or has anybody any idea of what his work sells for.