My dear Miss Marple, you have left a broad spectrum of suspects open to examination with this deduction.

As Mr. Watson was suffering in bed with rather unfortunate bout of influenza, I suspect it is unlikely that he was in fact posting as Thornton Curtis on saturday night.

It may have been the founding member of Rural Sex, but as I have no information relating to his his whereabouts on said saturday evening, my speculation is, as yet, unfounded.

Could it, in fact, be Henry, the mild mannered janitor?