The Grey Book is in its 50th edition, edited by Dr Teck Khong (firstname.lastname@example.org)
It was first published and edited by Professor Joe Collier in August 1979 and is probably the oldest established set of such guidelines in the UK.
It is available as a single PDF document from the Library E-books web page:
The Trust Intranet publishes the Grey Book as separate topics, together with evidence based references. Hard copies are available as usual from the Registry. The Library also holds reference copies.