Death in Desolation (Chief Inspector Littlejohn #45)

From the blurb of The Thriller Book Club edition of 1967:

One morning, a haystack is seen to be on fire at the desolate farm of Great Lands and when the police go to investigate, they find the bodies of Harry Quill, the owner, and his wife lying on the doorstep. He is dead and she is unconscious. Mrs. Quill, an invalid, had apparently burned the stack to give the alarm. She died in hospital later without recovering consciousness.

Chief Superintendent Littlejohn and Inspector Cromwell become involved in the murder case because they are already engaged on a nation-wide investigation concerning the Black Lot, a gang of criminals raiding isolated farms. However, on the very night of Quill's death, the Black Lot are arrested in the North of Scotland.

The Quill family is old and widespread, a closed shop to all except its members, who know all its secrets and discuss them freely behind closed doors, but are silent about them in public. They are headed by Aunt Clara, a very formidable old lady whom they all respect and greatly fear. Littlejohn finds himself face to face several times with Clara Quill and has several battles of wits with her and her crafty lawyers before he finally finds out who killed Harry.