Sarah Siddaway is a non-canonical character created for the 1993 film adaptation of The Case of the Missing Will in Series 5 of ITV's Agatha Christie's Poirot. She is the wife of John Siddaway and mother of Robert. The family are close friends of Andrew Marsh.
Investigations by Hercule Poirot established that Sarah was the culprit behind the string of criminal events in the show. Some ten years earlier, in 1926, John Siddaway as Andrew Marsh's solicitor, had persuaded his client to make a will. This he did, leaving bequests to various people in his circle of friends. These included a sum of money of £500 for John's son Robert to be held in trust until he was 18 to start him on his way in life. This however was not enough for Sarah. She wanted her son to have everything and schemed to get access to the bulk of Marsh's estate. When in 1936, over a lunch, Andrew Marsh mentioned to these same friend that he intended to change his will, Sarah was concerned about the implications. As a former children's nurse, she had learnt from her contacts that Violet Wilson was Andrew's illegitimate daughter, the offspring of a union with Phyllida Campion. Sarah was concerned that Andrew might be planning to change his will to favour Violet.
Sarah moved quickly to kill Andrew before he could change his will. As a former nurse, she knew the effects of injecting an overdose of insulin. She stole a vial of insulin from the bag of Dr Martin Pritchard. She then summoned Andrew by a phone call to an urgent meeting at a folly in the grounds of his house and killed him by injecting him with the insulin. At a later time, she then planted the empty vial there to implicate Pritchard.
Sarah then stole Andrew Marsh's 1926 will and destroyed it. This ensured that Andrew was intestate. She then hinted to her son Robert that Andrew might have been his father.
However when Sarah overheard that Phyllida Campion planned to go to London to see Poirot, she was concerned that Phyllida would reveal that Violet was Andrew's daughter. Sarrah followed Phyllida to London and pushed her down the escalator at a Tube station. Phyllida, however, survived and Poirot was able to retrace Sarah's tracks and unmask her as Andrew's murderer.
Sarah Siddaway is portrayed by Rowena Cooper.