Yes, that "William" is from marriage records. We know absolutely nothing of her parents or siblings. A witness at that marriage was "Joseph Newbould" - we think he may have been a brother or Uncle.
They married in 1874, and I was really surprised not to have been able to trace her, as she died comparatively (!) recently in 1933 in Southport.
I suppose your suggestion concerning the marriage certificate, not taking it as read is possible. I know names may be invented, or truth skated over, for propriety, on such documents, often happens - just I've not yet found any on that side of my researches that failed to check out as fully legit.
Ages back, I thought I "had her pinned", but that Ann Newbold was a Yorkshire woman herself, rather than just living there at the time of her marriage, and we had to eliminate her after accounting for her in the years that our Ann Newbold (Isherwood by then) was safely producing the next generation in Southport, and firmly insisting she had been born "Derby" 1855 in each census.
I found a lot of William Newbold / Newbould people listed in the Derby area, but cannot link her to any, so I'm more than a bit lost now. I'd hoped to trace a William Newbold with a daughter Ann ( or variants) in the 1861 census, at least, to give me a clue to siblings and both parents, but haven't managed a likely one, even without a convenient Joseph.
I did wonder about seeking her in non-conformist baptism records, as her husband became very involved with the Zion (Methodist?) chapel in Sussex Road in Southport, later, although in earlier generations the family seemed to be linked to the Anglican churches in town, but have no knowledge of how to access online any non-conformist baptism records for Ann in Derby, either.
Actually, looking at a local Southport newspaper account of his in 1925, funeral it gives their Golden Wedding Anniversary as having been celebrated the previous 21st Sept: "his death took place 12 months afterwards on the very date on which the anniversary of his 50th year of happy wedded life was celebrated" - and that gives the date of Sept 1874, correct for the marriage, and her age then was 19. So that checks out again with the age she consistently claimed to be.
Thanks for the thought. I just wish I could find her in her youth.