Script versus short story
The script stays remarkably close to its source material, which is hardly surprising given the fact that Exton adapted it (and the fact that the short story is itself an excellent one, with little need for alterations). The few additions include an opening sequence at Farley's factory, in which a new extension is opened, the introduction of Miss Lemon, Captain Hastings and Chief Inspector Japp (unsurprisingly). Exton makes a few sensible additions to the solution, too, including a massive wall outside the office windows (no witnesses), Farley checking up on his employees (a much better reason for coming to the window than in the short story), and a subplot for Miss Lemon (desperately asking Poirot for a new type writer - and providing Poirot with the vital clue of the window).
The introduction of the regular cast mostly makes sense here. Inspector Japp simply replaces the short story's Inspector Bartlett, Hastings gets very little to do anyway (he reads Poirot the letter from Farley, follows him to the factory - but isn't allowed to enter - demonstrates the murder in the denouement scene and takes on one of these ridiculous chase scenes (really, why do they have to add those all the time? I realise it increases tension somewhat, but still!). Miss Lemon would be the one to receive Farley's letter, so the inclusion of her character does make sense. On a side note, the doctor in the case, Dr. Stillingfleet, became a regular in Christie's stories, but he did not appear again in the television series.
All in all, the script is an excellent achievement, staying true to Christie while making sensible and small alterations. Possibly the best episode in this series (plot-wise, that is).
Directing, production design, locations and soundtrack
Bennett makes excellent use of the magnificent location and sets. I particularly like the outside shots of the factory in the denouement scene and the sweeping crane shot from Miss Lemon's window in Whitehaven to the street level (which must have been shot while they still had access to the entire building). The location used for the factory is the Hoover building in Perivale, Middlesex (which was also used as the film studio in The King of Clubs). Gunning's soundtrack (and the orchestra in the opening scene, in which he himself features!) is good, but not as good as some of his more memorable scores.
Actors and characters
(Miss Lemon mentions her magnetism in this one - a sign of the interest TV-Lemon has in alternative medicine, the occult and the mythological). The standouts here are Alan Howard (who brilliantly portrays both Cornworthy and Mr. Farley!) and a young Joely Richardson (as Joanna Farley).