One question that might be asked is: when did Martin Earl submit the advertisement to Bookdealer?
Well Bookdealer was published every Thursday and the instruction to advertisers of "Books Wanted" was this:
"Lists to appear once only should be addressed to Bookdealer, PO Box 1082, Winscombe, Avon BS24 6BX to arrive first post Wednesday for inclusion in the next week's issue."
What I think this means is that to get his ad into the issue dated 19th March 1992, Martin Earl needed to get his list to Bookdealer by first class post on Wednesday 11th March 1992. This in turn means that the latest he could have sent his list by first class post would have been on Tuesday 10th March 1992.
Given that Mike Barrett spoke to Doreen Montgomery on 9th and 10th March, then if we assume that the advert was placed in response to his conversations with Doreen, Mike wasted no time in contacting Martin Earl. In fact, if I have correctly interpreted the Bookdealer's instructions, he must have done so almost immediately after speaking to Doreen.