Script versus short story
To say that this is an adaptation of a Christie story is to stretch the meaning of that term particularly far out. In fact, there is almost no point in recounting the changes made to the story, because, essentially, the only elements kept are 1) a missing second will, 2) the names of the central characters, 3) the education / women plot point. There's no old farm, no quick-witted uncle, no search for a cleverly hidden will. Indeed, anyone claiming that this series never used to take liberties with the source material in its "golden years" should have a closer look at this episode. However, I personally think this works as an hour of television entertainment and as period drama. The extravagantly staged Cambridge Student Union debate on women and higher education is just one example of how Watkinson creates a very distinct 1930s university atmosphere. Also, the whole idea of different versions of a will, with several possible heirs and suspects, is very Christie-esque. Even the murder method, poison, seems as if it was taken straight out of a Christie tale. All in all, then, I'm not as outraged by this episode as I'm sure some purists would be. Poirot is his usual self, so is Hastings, Japp and Miss Lemon. It's also nice to have Poirot as a long-term friend of Andrew Marsh, a clever reference to Poirot's many friends and acquaintances, many whose deaths he will have to investigate in later years. Of course, one might easily object that there is a perfectly acceptable Christie story that could have been used, but I very much doubt that the somewhat slight plot of the original could have been expanded into more than fifty minutes of television. For that reason, I'm inclined to support Watkinson's (and probably the producer's) decision to create something entirely different. Especially since it generally seems to work.
Directing, production design, locations, soundtrack
Bruce's direction is competent, and he manages to bring out the university setting to a tee. The production design of this particular episode is really extraordinary - they manage to convince viewers that we are really seeing 1930s Cambridge. Locations used include St. John's College, Cambridge. Gunning's score is sufficiently evocative of the setting, too.
Characters and actors
As already mentioned, the main characters do seem at home in Watkinson's more or less original plot, and that's a big relief. Of the guest performances, no one except Beth Goddard (Violet) really has time to excel. As a curious coincidence, Goddard re-appears later in the series' run as a completely different character in another Christie story that is almost completely reworked, i.e. Appointment With Death.