Sheila Bowler spent four years in prison after being found guilty of pushing Florence Jackson into the River Brede near Rye in East Sussex in 1992.
Mrs Bowler always maintained her innocence. After two Appeal Court hearings, her conviction was quashed and a retrial ordered.
New medical evidence had emerged, not put before the original jury who had found her guilty by an 11 to one majority at Hove Crown Court in July 1993.
Retrial reveals lack of evidence
The prosecution alleged that Mrs Bowler, from Rye, had murdered 89-year-old Mrs Jackson in May 1992 while driving her from a residential home to her own house.
Prosecuting, Anthony Glass QC alleged that Mrs Bowler killed Mrs Jackson on the journey then covered up her deed by pretending her aunt - who normally needed help to walk - must have made her way to the river and accidentally fallen in.
However, Mrs Bowler said she had left Mrs Jackson in her car when she went to get help for a flat tyre. When she returned her aunt had disappeared.
Defending, Jeremy Roberts QC said the prosecution "had not produced one shred of direct evidence to connect Mrs Bowler with whatever it was that happened to Mrs Jackson that night".
He told the jury: "No witness claims to have seen Mrs Bowler or her car at the pumping station or in Station Road that night. There is no scientific evidence suggesting Mrs Bowler had ever been in that area."
He said that the circumstances in which Mrs Jackson died "were likely to remain a mystery to which none of us will ever know the answer".
The court heard that Mrs Jackson was the aunt of Mrs Bowler's late husband, and that her only asset was a flat in Rye, which she was leaving to her niece.
Mrs Bowler had power of attorney and was responsible for arranging the payment of fees at Greyfriars, a residential nursing home at Winchelsea where Mrs Jackson lived.
Prosecuting, Mr Glass alleged that Mrs Bowler had a financial interest in Mrs Jackson's death. But Mrs Bowler said that she received £17,500 a year from teaching at private schools and pensions, the mortgage on her home was paid off and she had savings.
The jury returned an unanimous verdict of not guilty.
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