…well, ok, great if you’re an Anglo-Saxon liturgy geek… (so maybe “invaluable” is a little extreme, but since I don’t have consistent access to Milfull you have no idea how helpful this is to my dissertation.)
bls directs us to the Anglo-Saxon Hymnarium produced by the Surtees Society under the editorship of the Rev. J. Stephenson reprinted as the volume for 1851. (Here’s the alphabetical index if you want to check for any particular hymns. [Important note: this text contains only the Latin and the Old English gloss. It does not contain modern English translations/paraphrases/equivalents.])
What this means is that yes, it contains a transcription of the Durham Hymnal; no, it does not necessarily follow current editorial standards–caveat lector! So, for basic information this is a great reference to have sitting on your hard-drive; for academic citation, go look it up in Milfull first.
This is also helpful and fascinating for those with an interest in the history of the Ritualist/Anglo-Catholic movements. In terms of “what did they know and when did they know it”, this date establishes the available presence of a classical Anglo-Saxon hymn cycle before the first publication of Hymns Ancient and Modern (TOC here) which first appeared in 1861 (in planning since 1858) and which included some Anglo-Saxon options in the Sarum Office Hymn list of 1904.
For the Aelfric folk in the crowd, there are some interesting connections between the Durham Hymnal and Aelfric. For instance, I believe that the Hymnal was bound with an edition of Aelfric’s Grammar—which may make the glossing that much more interesting since his grammar included a glossary (a list of Latin words and their Old English equivalents). When the two texts were bound together I cannot answer and should look up… Furthermore, the order of hymns in this hymnal can be compared with the list that Aelfric gives in the Letter to the Monks at Eynsham the temporal cycle of which I mostly reproduced here. IIRC, they are similar but by no means identical (reminding us once again of the inevitable variation in medieval liturgy).