The Sarum Rite of the Western Church grew up through the period 1000-1500, and was used throughout much of Britain and parts of North-Western Europe. Sources for this rite exist in a considerable number of manuscripts as well as a large number of printed editions dating from the late 15th and early 16th centuries. Under the rule of Elizabeth I, the Sarum Rite was finally abolished and replaced (in England) by the Book of Common Prayer. In the past two decades, Nick Sandon has published the greater part of the music of the Sarum mass in a fine series of editions published by Antico Music. The Gregorian Institute of Canada has now begun the publication of the Music of the Sarum Office, containing the full text and music of all the services of the office for all Sundays, Week-days, Feasts and Fasts of the Year and Saints Days. This edition, comprising approximately 5000 pages of text and music, will in the first instance be published serially in PDF format. Each installment, which will contain approximately 250 pages of the edition, will be reviewed for content and accuracy by an Advisory Board before publication. Publication began in January 2006. New installments are published every six months.
Here's the downloads page, where there have been recent revisions and additions (although this is definitely a work-in-progress, as many files have not yet been added). Here's what the intro paragraphs there say:
The following files are or will be available in .PDF format, beginning in January 2006. Adobe Acrobat Reader is required to read and print them. Each file name is followed by its original and most recent upload dates. Unlinked files are in preparation.
Since the spring of 2007 there has been an entire revision of the Psalter [A-1 through A-12] and the Temporale [B-1 through B-5]. This revision includes accents for all sung or spoken texts as well as references to the index numbers of the CANTUS database of Latin Ecclesiastical Chant.
P.S.: In case you're interested, here's a little PDF pamphlet titled "The Sarum Use," written "By the Reverend Canon J. Robert Wright, Historiographer for the Episcopal Church" - which, it says, was "An unpublished address given at the Miller Theatre, Columbia University, on January 26, 2002." Don't say I never gave you anything.