Wellcome Trust have a well established policy requiring research papers written by their grant holder to be open access. They are now extending this requirement to research published in scholarly monographs and book chapters as well. See the Wellcome Trust press release for full details.
HEFCE's consultation on open access requirements for the REF post-2014 invited comment on whether it would be appropriate to expect monograph content submitted to the REF to be open access. We're still waiting for the results of that consultation. Meanwhile, I'm eager to see how the Wellcome Trust requirement fairs: How easily will it be for grant holders to comply? Will it encourage more open access options for researchers?