The fact that the Psalm 21 initial is enlarged (three lines in height, instead of Psalm 20's two-line height) suggests a Benedictine connection: Psalm 21 was one of the Benedictine divisions of the Psalms, being the first Psalm said at Matins on Sundays.
The leaf recognisably comes from a very handsome but somewhat defective volume of 129 leaves offered by Maggs Bros., Catalogue 1262: Illuminated Leaves and Historical Documents (Christmas, 1998), item 7, and again in European Bulletin, no.23: Illuminated Miniatures, Manuscripts, and Single Leaves 1000-1900 A.D., Catalogue 1366 [December, 2004], item 1, with a colour reproduction:
The Maggs description reports that it was previously Christie's, 2 July 1975, lot 290, when it had "140 leaves (including one replacement leaf at the beginning)".
Of the leaves that had been removed from the volume between 1975 and 1998 were:
- Maggs Bros., Bulletin 12, no. 25
- Maggs Bros., Bulletin 13 (July 1986), nos. 32, 33
- Maggs Bros., Bulletin 15 / Catalogue 1103, no. 55
- Maggs Bros., Bulletin 22 / Catalogue 1249 [April 1998] no. 4, with reproduction. Pss.17:39-18:7
- Sotheby's, 17 June 1997, lot 20 (a)
EDIT 25 April 2014
I have just noticed that the volume was sold at Sotheby's, 23 June 1998, lot 45, with a description listing several single leaves that had been extracted since 1975.