I’ve updated the Thomas and Sarah Families paper yet again because looking more closely at the census entries for James, the eldest of the Walmersley children, I see he has a Sarah Jackson living with him in 1841. As a widowed Sally Warburton married James Jackson in 1818 this is probably his mother; the 1841 census doesn’t state relationships.
This doesn’t prove she is Sally Isherwood, though her probable death in 1846 aged 69 does fit with the baptism of a Sally Isherwood from Bashall. This is some 18 miles north of where Sally first lived with Thomas at Hall Carr, and while she could have moved for work, the match could be a coincidence, or her age slightly wrong. I have yet to determine if any of three adult Thomas’s who were buried in the Bury area between 1805 and 1818 could be an alternative husband for her.
On balance this new information strengthens the case that Sally Isherwood was the mother of the four Walmersley children.