Accordingly, next time I was at the university library doing research, I dug out the Waugh biography given in the book on Renault as the source of the quote,* and found out it was a false lead. Waugh did indeed express that opinion of certain Oxford types, but he didn't name Lewis or Tolkien: it was Renault's biographer who decided L and T were among the people Waugh was criticizing, and put their names into the context. Which might well be the case, but if so it was a general, not a specific or personal, criticism: the Waugh record remains, so far as I know, Tolkienless.
However (and here's where the nugget of copper comes in), a second Waugh biography I checked just in case** revealed the unexpected information that young Waugh was a great admirer of James Branch Cabell, and that his "first serious attempt at fiction" was heavily influenced by J.B.C. Here's the passage in question:
The result was Evelyn's first serious attempt at fiction. It was a macabre story of death entitled Anthony, Who Sought Things That Were Lost. It was set in a tyrannical grand dukedom of Italy in the year 1848. Harold [Acton] published it in The Broom in June 1923. Fears aroused by the preciosity of the title are fully confirmed by reading.
Of this juvenile work Evelyn wrote that it 'betrays the unmistakeable influence of that preposterously spurious artefact, which quite captivated me at the age of nineteen, James Branch Cabell's Jurgen'. The severe self-judgement must stand, but it is to be noted that Harold, according to his own account, had already begun to convert Evelyn from Cabell towards a worthier model, Ronald Firband. Evelyn's story, gruesomely telling of an imprisoned pair of lovers, and the decay of their passion amid the horrors of the dungeon, and of how the love of 'the Lady Elizabeth' was transferred from 'Count Anthony' to the jailer, and of their murderous end, certainly shows the rubbishy influence of Cabell; but others are evident as well. There are weak echoes of Oscar Wilde's 'Happy Prince' stories, of Edgar Allan Poe, of Firband. Oddly enough the strongest resemblance is to an author whom neither Evelyn nor Harold is likely to have read or even to have heard about then, Frederick Rolfe, 'Baron Corvo'. At that time he was completely forgotten. The story is not by any standards a good piece of writing . . .
[Sykes, p. 47]
I have to admit I've never read any Firbank -- an omission wh. I shd perhaps rectify -- but I do take exception at Sykes' snide dismissal of Cabell, whom I consider a much better writer than Waugh himself. Not that the synopsis given sounds anything like JURGEN, which is worldly and witty, cynical and salacious. Cabell had his limits, and doesn't lend himself well to imitation, but he also had real talent.
The main surprise is that Waugh would have chosen a famous fantasy author as a role model, and an American at that. As with Dunsany at about the same time, it's easy to forget today how popular, and admired, Cabell was as a writer in the late teens and early twenties of the last century.
By the way, it's my understand that Tolkien did read JURGEN and didn't like it (cynical, salacious, and irreverent not being exactly his cup of tea); Lewis also read it (probably at the urging of Joy Gresham***), as is shown by various passing allusions to the book (e.g., AN EXPERIMENT IN CRITICISM, p. 50), but I can't recall his discussing or evaluating it.
So: a fairly typical mainstream dismissal of a fantasy influence on a major twentieth century British author, but of interest for what it reveals about said fantasy author's influence and wide appeal at the time.
current reading: SPEAKING FROM AMONG THE BONES (2013)
*EVELYN WAUGH: THE EARLY YEARS 1903-1939, by Martin Stannard , pages 85-86
**EVELYN WAUGH: A BIOGRAPHY, by Christopher Sykes .
***Joy G. being an admirer of both Cabell and Dunsany.