Often the primary home for classic British mysteries like this. You can check availability on BritBox.
High-production values capture the glamorous aesthetic of the late 1920s.
If you are looking for or ways to watch the series, here is the current status: Where to Watch
The series, which includes the pilot movie "Speedy Death" and four subsequent episodes, is available on several major platforms:
Streaming options may be available for Roku users. Subtitle Availability
Diana Rigg brings a "Dadaist" flair to the character, breaking the fourth wall to share witty insights with the audience.