The reference comes in STUDIES IN THE HISTORY OF OLD ENGLISH LITERATURE, a collection of essays and articles by Kenneth Sisam, Tolkien's old tutor, in the third piece, entitled 'Seasons of Fasting'. Here's the passage referencing Lord Howard de Walden in full:
When he was examining the transcripts of Laurence Nowell
which Lord Howard de Walden gave to the British Museum,
the late Robin Fowler found in MS. Addit. 43703, transcribed
by Nowell in 1562, a copy of a poem on the observance of fasts
which, except for the incipit recorded by Wanley, had been lost
when MS. Otho B XI was burnt in the Cotton fire of 1731.
Flower's death prevented him from making the edition he planned,
and it fell to Professor Dobbie to publish the editio princepts
in his Anglo-Saxon Minor Poems, 1942 . . .
This notice is also interesting because it brings together, briefly, two underappreciated figures: Lord Howard de Walden and Laurence Nowell. Nowell is one of the unsung heroes of Old English studies, whose greatest claim to fame is that he preserved BEOWULF: the first page of the manuscript that includes the only surviving copy of BEOWULF bears the inscription "Laurence Nowell 1563". We have no idea where this manuscript (now called THE NOWELL CODEX) was before Nowell found and preserved it, a full century and a half before it fell into Sir Rbt Cotton's hands.*
Nor is this all: Nowell also compiled the first OLD ENGLISH dictionary, which I have a copy of: VOCABULARIUM SAXONICUM, probably compiled in the 1560s but not published until 1952.
And furthermore David Salo believes, rightly I think, that Nowell's dictionary had a direct influence on Tolkien. For example, the word 'orc' is usually translated 'monster' and associated with the demon Orcus (a derivation Tolkien is on record as doubting**): Nowell's entry is far more evocative of Tolkien's own usage:
Orc. Orcus; a goblin, a Robin Goodfelowe. Entas & eotenas & orcas. [Nowell, p. 134]
So, there it is, dating all the way back to Tudor times: an association by a great Old English scholar of orcs not with Latin terms for Hell but with native faerie lore: goblins. Very Tolkienesque, is it not?
*here's a query for the well-informed: is there a complete catalogue listing all the books in Cotton's library, a brief description of their contents, and a notice accompanying each entry on whether or not that particular book survives?