What an intriguing mystery!
Just catching up, reading through old articles, etc. I think all the theories in this thread are excellent possibilities, though I am 100% convinced that "Jestyn" was definitely the key to this case.
In the Wiki article there was mention that "in late 1948 a mystery man had asked her next door neighbour about her".
From that point, for this man to die in this manner (in the town Jestyn was currently living), and later have the text "Taman Shud" found in his pocket, torn from the page of a book that contained Jestyn's phone number in it... and not just any book- but a copy of the same book that Jestyn just happened to have given a male army lieutenant a few years prior- is just too much coincidence for me.
The police officer that questioned Jestyn made notes in his notebook that Jestyn appeared ready to faint when asked to identify the bust, though she ultimately proved unable (unwilling??) to say who it may be.
THEN, factor in that Jestyn had a son who just happened to have
not one, but two of the extremely rare genetic conditions- anadontia AND an ear condition where "the cymba (upper ear hollow) is larger than his cavum (lower ear hollow), a feature possessed by only 1-2% of the caucasian population"- that the Somerton man also had. What are the odds of that?
Investigators have a ton of information about all kinds of non-essential clues, but almost no information about the one person who almost definitely held the key to this entire case- Jestyn.
She had extensive ties to the military, so is it impossible to imagine she orchestrated- or at least had intimate knowledge of- this man's death? I am convinced she knew who the man was, and recognized him on sight. I also believe she knew who the "stranger" was who had inquired about her the year before the Somerton man was found dead. In addition, she very well may have remained in contact after the war with a military person that possessed the knowledge and resources to poison a man with a undetectable substance.
Perhaps she heard, through her own sources, or from correspondence from the Somerton man himself, that he was to be in town the night of November 30th. Maybe she was scared that he'd try to come by, as he'd tried to do the previous year.
Her motive was likely to protect a secret- her son was fathered by another man (the Somerton man), and not by her husband. Women do crazy things even today to hide those sorts of secrets... and we live in a much more liberal society than was present in 1949. Her son would have been a toddler at the time of the Somerton man's death.
I would love to have some information about Jestyn's husband. Was he a military man too? Did he find out that the son he thought he shared with his wife wasn't really his? Perhaps he orchestrated this, leaving cryptic clues behind that would mean nothing to anyone but Jestyn. Maybe he killed this Somerton man out of jealousy/anger/to keep him away from Jestyn's son/to protect the family secret about the love-child, and wanted Jestyn to know what he had done, even as the police couldn't tie it anything. Using the "Taman Shud" from that particular book would have told Jestyn why this murder was committed (because of her history with the text), but wouldn't have led police anywhere as far as solving the crime was concerned.
That might also explain Jestyn's demeanor when being questioned by police... as well as her request to remain anonymous. She was obviously concerned with appearances and her reputation. A love-child/love-triangle would have ruined her.
JMO
http://en.wikipedia.org/wiki/Taman_Shud_Case