I have tracked down the advertisement placed on behalf of Mike Barrett for an unused or partly used diary dating from 1880-1890 which had to have a minimum of 20 blank pages.
It was placed by Martin E. Earl in Bookdealer, the trade weekly for books wanted and for sale, issue No. 1044, dated 19th March 1992. An image of the advertisement is reproduced below.
The request for this diary appears in a long advert - in the BOOKS WANTED section - with a further (by my count) 86 requests by Earl for various books (one can see a couple more in the below image) in a total of (by my count) 104 lines. The way it worked was that bookdealers were given a special rate of 22.5p per line. Given that the ad for the diary was over two lines, it would have cost Martin Earl only 45p to place the ad for the diary, which I assume was included in his margin when he sold the 1891 diary to Barrett.
On that basis, it seems likely that Mike Barrett never even knew of the existence of this advertisement (and was probably never asked to pay for it). It was simply a cheap way that a bookfinding company like Earl's would find obscure books on behalf of its clients.
In 1992, Martin E. Earl was based in Oxford. His address and two telephone numbers were provided at the top of the advertisement. The company trading as Martin E. Earl in 1992 appears to have become H.P. Bookfinders in 1995.
According to the Casebook transcript, Mike Barrett in his affidavit of 5 January 1995 said that his wife used "a firm in the 1986 Writters (sic) Year Book" to find the diary (although he could not remember their name). By this he must have meant The Writers' & Artists' Yearbook 1986 (which is certainly how Shirley Harrison transcribes it in her book).
However, The Writers' & Artists' Yearbook 1986 does not include entries for any bookfinding companies (so that there is no entry for Martin E. Earl or, obviously, H.P. Bookfinders). Consequently, Martin E. Earl's details were not found by either of the Barretts in there. However the contact details of Rupert Crew Limited (of which Doreen Montgomery was a director) and also of Pan Books, which Barrett is supposed to have contacted before contacting Doreen, are both to be found in the Yearbook. Perhaps Barrett got confused when he came to write his affidavit.
We can be certain that either Mike or Anne DID locate and contact Martin E. Earl in March 1992 so that the error with recalling how he (or she) did so would appear to be a good example of an innocent mistake of recollection.
Finally, I can confirm that in March 1992 Outhwaite & Litherland held auctions once a week, every Tuesday, so that the first auction held after Barrett would have received the 1891 diary would have been on Tuesday, 31 March 1992. The auction (like other auctions in that month), held at Kingsway Galleries, Fontenoy Street, Liverpool, was described in antique magazines as being for "Victorian, Edwardian & modern furniture and effects". It started at 10.30am. Had Barrett taken 11 days to forge the diary as he claims in his affidavit and, had he started work on 31 March, the writing would have been finished on 10 April. He went to see Doreen in London on 13 April.