Late 1930s, early 1940s.
This seems to be a pretty early Maudella offering. In the United States this would almost certainly be called a "gymnasium suit," and might be worn with a white blouse underneath.
This pattern may have been offered in response to movement started in the 1930s to promote fitness for women. It's tempting to imagine Maude Dunsford reading about (or even being a member of) the Women's League of Heath and Beauty, founded in 1930 by Mary Bagot-Stack and in 1935 carried on by her daughter Prunella. (1)
Click on the image to watch a lovely 1930s British Pathe film of League ladies going through their paces.
This pattern is unprinted but each pattern piece is stamped with its name. There is no separate instruction sheet, only the text instructions on the back of the envelope.