Field’s conviction had been referred by the Criminal Cases Review Commission under the exceptional circumstances provision, which allows a new appeal even if there is no new evidence.
Reading a summary of their ruling, the appeal judges said the jurors at trial had “not been properly directed” and the directions given to them on how to reach a verdict were “defective”.
Lord Justice Edis said: “The directions effectively withdrew from the jury the question of whether Mr Farquhar’s decision to drink the whisky had been voluntary.”
Alongside his life sentence, Field was given a concurrent 16-year jail term for admitting fraud and burglary offences in relation to Farquhar and a neighbour – Ann Moore-Martin, 83.
Field, from Olney in Buckinghamshire, had also targeted Moore-Martin, a retired headteacher who he manipulated by writing messages on her mirrors purporting to be from God.
The former church warden admitted fraudulently being in relationships with the two pensioners as part of his plan to get them to change their wills.
He was found not guilty of conspiracy to murder Moore-Martin and an alternative charge of attempted murder. Moore-Martin died of natural causes in May 2017.