Isabella Rayner was the sixth child and third daughter of William Rayner and Susannah Chapman. Isabella was married twice and had ten children that have been traced.
Isabella Rayner, William Nichollias and Joseph Knight
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Identity of Mary Nicholls
Mary Nicholls was probably the Mary Agnes Bottriel who was married to Peter Leslie Nicholls, a descendant of William Nichollias and Isabella Rayner. In addition to the works mentioned previously that were authored by Mary, Michael Roe mentions a Mary Nicholls in relation to his research into The Tichborne Claimant, noting that Mary had added significantly to the story.1
Mary's life was succinctly reported in the Papers and Proceedings of the Tasmanian Historical Research Association of which she was a long term member. The article confirmed that Mary worked at both the University of Tasmania and the Archives Office of Tasmania, before retiring in 1988. Mary Nicholls died in 2009 in Hobart, Tasmania.2

A previous version of this
Early Nichols-Nicholls-Nicholas Researcher
There is an interesting aside to the Nicholls family history. One of my earliest road trips in relation to the family was to the Friend's School Archive where I had heard there was a copy of the original wedding certificate for William Nichollias and Isabella Rayner. Ultimately I was told that the item I was after was actually in the Society of Friends Archive at the University of Tasmania, but the staff kindly provided me with a one page typed paper they had on the Nicholls family named Nicholls-Nicholas-Nichols. I reviewed the original certificate at the University, and they also had the early minutes of the Society of Friends. After a long process of transcription some of this material, and the earlier information from the school, was included in the story of Isabella Rayner and William Nichollias. After I published the page on a website I was contacted by a fellow Nicholls researcher Marian McColl who related that:
We can only wonder where this goldmine of Nicholls information is sitting now, or if it still exists. I trawled the University Thesis Index and there were no matches for a thesis by Mary Nicholls. Some of the staff at the Archives Office remembered Mary but she had retired 10 years earlier. Unfortunately Mary passed away ca. 2008-2009. This comment was initiated when the other day I saw a copy of a transcription of Knopwood's Diary and it was by Mary Nicholls. The AOT staff confirmed that the same Mary Nicholls who worked there was the author of that book, and they also confirmed the same person was researching the Nicholls and Rayner families. Given her background it seems odd that Mary didn't deposit her research with the AOT, it probably remained in her family as Mary had four or five children. Either way Mary will be remembered for making a significant contribution to researching the Nicholls and Rayner families, as well as to Tasmanian history generally.