The film is quite true to the macabre essence of Machen's story and develops it very well. Machen based the idea of the story on the Crippen murder only Machen regarding Crippen as rather foolish, describing him as "elementary Crippen and his bungling imbecilities", imagined a protagonist who was rather more careful. You can read the Islington Mystery here.
The IDMB database entry for the film is here:
Someone has put El Esqueleto de la Senora Morales up on Youtube. No subtitles but you can get a good sense of it.
[I have not updated this blog in ages. I will try and keep it more up to date in future and add in some details I missed.]