Perhaps a bit late, but the significant pieces of the entire piece, which is basically an entire service set to music, are found here.Clips of the Holden Evening Prayer
The Psalm 141 is, as Mr. Campin propounds, actually in Eb minor. This, most likely has to do with the fact that the "Thanksgiving Prayer" before it is in Eb, and the "Magnificat" after is in Ab.

As to being guitar friendly, it isn't hard to work out an accompaniment to it on the guitar. Put the capo on the six fret, and use the Am fingering for Eb minor, the C fingering for F#, the G major fingering for Db, and the D minor fingering for Ab minor.If there's a Bb minor, use the Em fingering, and you're done.