I looked into the Armchair Detective and learned he was the title character in a successful 1940s British radio series. Actor/writer Ernest Dudley (original name: Vivian Ernest Coltman-Allen) played the lead. Dudley also created the popular [i]Dr Morelle[/i] radio series. In 1952 Dudley starred in an Armchair Detective movie. Wikipedia suggests there was also a TV series, but I wonder if they haven't confused it with a 1949 American crime series with the same name.
The story is entertaining if a mite stuffy. The art suits it perfectly: well-drawn and a mite stuffy. I could picture this as a pleasant 1940s B-movie.
Link to the book:
Super Detective Library 008 - The Case of The Sinister Castle