Major Herbert Rowse Armstrong was a solicitor in sleepy Hay-on-Wye. He was a respected professional, churchwarden, family man and pillar of the community. Small and dapper, he lived in apparent contentment with his wife and three children in an imposing Edwardian villa with a large garden. And on 31 May 1922 he was hanged at Gloucester jail for murdering his wife by poisoning her with arsenic.

Was he guilty? The question is still open after a hundred years, and the case continues to fascinate. The Hay solicitor, quiet, ultra-respectable, the last person anyone would suspect, could have stepped straight out of a 1920s detective novel. There was a background of small-town rivalry (a new solicitor