Barnaby is quite right to rethink the "natural death" of Sir Freddy Butler from myocardial infarction whilst at Dinner with his family.
The "Mona Lisa' like enigmatic smile from Lady Annabel Butler in the police car, in conversation with Barnaby shortly after her arrest, did seem to indicate that there might more to his heart attack than met the eye.
Well I have done some research and I think that I can add some good circumstantial evidence that Lady Annabel did in fact poison him. Not only that, but I can tell you what she used as the poison.
The an extract from the widely distributed Afican plant Adenium obesum, was used as an arrow poison in Sudan, Somalia, Kenya, Tanzania, Cameroon and Nigeria.
The plant contains 30 cardio-toxic glycosides which act in a pharmacological manner to Digitalis. It acts on the Na+K+-ATPase enzyme that regulates the concentrations of Na+ and K+ ions in body cells and also modifies the Ca++ concentration.
In low doses,its western cousin Digitalis, is used to treat congestive heart disease and heart rhythm problems. In high doses it leads to systolic heart failure and death this can be anything from 2 to 8 minutes in humans depending on body weight.
The poison can be extracted and refined (I will NOT describe the process for obvious reasons!) to create an odourless tincture that is applied by a range of tribes including the Hadza to their hunting arrows. The plant can easily be dried and transported and stored. Usually wrapped in cloth to keep it away from daylight.
It is my belief that Lady Annabel, for some reason brought this plant back from Kenya. She extracted the poison and "at some point" managed to get it into Sir Freddy's body. Maybe before dinner in his drink.
There is no fault on George Bullard for failing to find the metabolites of this poison in Sir Freddy's system as he simply wouldn't have any reason to run that kind of toxicological screen given what appear to be the straightforward natural death of an elderly over indulgent person whose heart was not in a good state.
Lady Annabel using considerable cunning pulled off the "perfect" murder. However maybe her vanity overcame her she couldn't resist giving Barnaby that enigmatic smile when he re-stated what he believed to be the fact that Sir Freddy died a natural death.
If Barnaby asks Dr Bullard to run wide spectrum toxicological tests on the liver samples, it is possible that he will find the metabolites of this poison although only about 16% of this poison is likely to be metabolised. It has a half life of 1 to 2 days so after death, some digoxin-like metabolites may still be present in the liver after death.
I apologise for the length of this post.