Ah, I was right. The quote is not from Lewis, but from Hugo Dyson. And according to my sources in the Tolkien Society

there's some disagreement over whether the expletive was used or not.
Scull and Hammond in the
Reader's Guide offer the following, from A.N. Wilson's biography of Lewis:
Quote:
Originally Posted by Reader's Guide, p. 232
He [Dyson] was famously impatient with readings from The Lord of the Rings, then a work in progress, and was allowed to veto them when he was present [ie, at Inklings meetings]. According to A.N. Wilson, Tolkien 'could not be sure that his readings would not be interrupted by Dyson . . . snorting, grunting and exhaling --"Oh **** not another elf!' . . . . (The phrase is often mistakenly attributed to Lewis.)
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I think I shall be perusing S & H for evidence of any more Inkling humour.