HEFCE have set out proposals for implementing an open access requirement in the post-2014 REF.
The core of HEFCE's proposal is that journal articles and conference proceedings should be accessible through a UK HEI repository (like the University of Bath's Opus) immediately upon either acceptance or publication. Both 'green' and 'gold' routes to open access would be acceptable, as long as the accepted, peer-reviewed text is accessible through a repository. Embargo periods on copies in repositories are also acceptable.
HEFCE propse to apply this open access criteria to publications from 2016 onwards.
HEFCE's consultation is open until 30th October 2013 and they intend to announce their policy decision in early 2014.