Her age of 60 in the 1851 census could be wrong, if she is indeed the mother of Mary Phillips nee Bell (b. abt 1801).

There is an entry in the death index for an Esther Bell, 1853 Dec Liverpool.

From Chris Hughes, Jan 2008:
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I have got a copy of Esther Bell's death certificate from the GRO....

Registration District: Liverpool.
1853 Death in the Sub-district of Saint George, Liverpool, in the County of Lancaster.
No: 117
When and Where died: Ninth November 1853, 67 Cable Street.
Name and Surname: Esther Bell Sex: Female Age: 66 years
Occupation: Widow of George Bell, Warehouseman.
Cause of death: Bronchitis - 3 months - certified
Signature, description and residence of informant: Ann Bell - present at death - 67 Cable Street, Liverpool.
When registered: Twelfth November 1853. Thomas Parr, Registrar.
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This would make her born bet 10 Nov 1786 and 9 Nov 1787.