Hmmm. I'm beginning to understand why Tigus was wondering whether Crofts's insistence on playing fair with the reader is going to the disadvantage of Inspector French, in not making him draw conclusions and open up lines of investigation that would occur to a reader at this point.
Reading this for the Detection Club Bingo, and will also be posting a guess on its basis in the KYD green round (not saying yet for which card, though).