http://www.jenkins-ip.com/patlaw/cdpa1.htm#s163
Copyright ... Act 1988
Duration: 125 years after creation (for unpublished works)

Crown Copyright in public records
"Unpublished public records and those open for public inspection are reproducible freely under waiver of copyright. This guidance explains how this works in practice"
http://www.hmso.gov.uk/copyright/guidance/gn_03.htm

Copyright Policy of The National Archives:

"The Crown copyright protected material (other than the Royal Arms and departmental or agency logos) may be reproduced free of charge in any format or medium provided it is reproduced accurately and not used in a misleading context. The exception to this rule is for material downloaded from our DocumentsOnline service (see below). Where any of the Crown copyright items on this site are being republished or copied to others, the source of the material must be identified and the copyright status acknowledged.
Images on this site may not be reproduced without payment of a fee to the image library."
http://www.nationalarchives.gov.uk/legal/copyright.htm

See also:
http://www.lexum.umontreal.ca/conf/dac/en/sterling/sterling.html