Ch 3 states (Admiral Croft) accidentally hearing of the possibility of Kellynch Hall being to let, and understanding his (Mr. Shepherd's) connection with the owner, he had introduced himself to him in order to make particular inquiries

So I can only presume that Mr. Shepard had to discuss this with someone for some other person to be discussing it for AC to accidentally overhear it. Or is this all just euphemisms (for Sir W's benefit and in consideration of his nerves) for Mr. Shepard having approached AC. Since apart from the family, it was only LR and Mr. Shepard who were involved in the "retrenching plan" and LR is sooo considerate of SW's feelings, I do not see her discussing/gossiping about this. Mr. Shepard on the other hand could be seen as acting responsibly to carry out his charge even though he's gone against SW's wishes of it only happening if someone should request to let Kellylnch. I know this is a convoluted post and can only hope it makes sense!

I suppose the other possibility is that others know (or guess) about SW's financial state either due to lateness in payment or the mortgages themselves.