Her maiden name is SLATER on the birth records of her children, as per GRO.
Sources
"England and Wales Census, 1851," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:SGY7-7N1 : 27 October 2017), Mary Slater in household of John Slater, Ribchester, Lancashire, England; citing Ribchester, Lancashire, England, p. 21, from "1851 England, Scotland and Wales census," database and images, findmypast (http://www.findmypast.com : n.d.); citing PRO HO 107, The National Archives of the UK, Kew, Surrey.
John Slater and Mary Slater "of this parish" were married by banns at St Wilfred; unfortunately, no parents are specified.
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