Hi Bob
I think you'll probably have a problem tracking down a cheap copy, it was published over 10yrs ago. I'd suggest watching e-bay.
As an alternative there's the Brown's book, Prehistoric Rock Art in the Northern Dales, published in 2008 by Tempus (History Press), it covers the same ground and more. You can pick a copy up on amazon for about £13 or from the publisher for the full wack £19.
For reference there's the excellent ERA website http://archaeologydataservice.ac.uk/era/
and of course there's
http://rockartuk.fotopic.net/

cheers
fitz