Script versus short story
The adaptation stays largely faithful to its source material, since most of the added scenes are extracted from tiny references in the short story. There is, for instance, the opening scene displaying the Bond Street (here: Burlington Arcade) robbery and the funny scene with Poirot dressed up as a locksmith. Some sequences have changed location, such as the first meeting between 'Lady Millicent', Poirot and Hastings (now in a lavish hotel), the denouement (now in the Natural History Museum). Some elements are added, including the presence of Miss Lemon (who doesn't get much to do) and Japp (who gets to bail Poirot out of prison (!) and chase the culprits (yes, there's an added chase scene as usual, but this one makes excellent use of the location!). My main criticism of the script is that a large section of Poirot's explanation is cut. How did he know that Lady Millicent was not Lady Millicent? How did the culprits know about the hidden Chinese box and the letter? All of this is lost in the adaptation and could actually leave viewers confused as to what had actually occurred.
Directing, production design, locations, soundtrack
Edward Bennett does a competent job of showcasing the locations. This episode is yet another example of the extravagance of the production team. In scenes that could easily have been depicted in low-key sets, they go for large hotel lobbies (a.k.a. Senate House, London) and famous places like the Burlington Arcade and the Natural History Museum (see Joan Street's website for photos). The soundtrack for this one is magnificent (again, by Fiachra Trench), and I really wish it had been released on CD.
Actors and characters
Poirot's wonderful disguise is quite out-of-character in a way, but at the same time, he is known to resort to made-up identities, and the whole locksmith business does sort of take place in the short story (he doesn't seem to be disguised there, but such an addition makes complete sense).
Of the guest actors, Frances Barber (Lady Millicent) really stands out with an attention-capturing performance, but so does Carole Hayman (Mrs Godber) as the housekeeper (who doesn't quite welcome Poirot 'with enthusiasm', as in the short story!)