Thanks for this information and sorry for initial confusion. It is a complicated story and I was trying to keep it simple!
John's mother Jean Buick Whitton had a colourful life and children I believe with three fathers, apparently with older men for whom she was working as a servant. She was baptised at Arbirlot on 21 May 1822 and had a son William with George Livingston(e) on 14 Sept 1843. She then apparently married Edward Johnstone in Dundee on 25 August 1851 (this is confirmed when her son William's marriage in 1860 gave his mother's name as 'Jane Whitton now Johnstone'.
In 1855, Edward and Jane had a son, registered 18 Dec 1855 in Dundee. Unusually, this son had no christian name. I believe this is the person known as John Robertson (see below). On 29 October 1857, Jane/Jean had a daughter Margaret Whitton, registered as illegitimate.
We then come to the Robertson connection. When John Robertson was married in 1888 his parents were stated as Thomas Robertson Road Contractor and Jean 'Carey' Whitton. John's age was given as 32. When Thomas Robertson made a deathbed will in 1879 he left his entire estate to Jane Whitton of 43 Union Street Dundee, and his executor was John Robertson, Law Clerk. Thomas's normal abode was Perth and I think the Union Street address was Jean's. (Incidentally, in another will, of her uncle in 1878, she was said to live at Elsinore Place, Victoria Road, Dundee).
I believe Thomas and Jean never married and that both John and Margaret were their illegitimate children. My theory is that John is the unamed boy registered to Edward Johnstone and Jean in 1855. Although his age on census would suggest 1857 and his age at marriage would suggest 1856, I can't find any feasible alternative. Probably the marriage of Jean and Edward was short-lived and broke up when she formed a relationship with Thomas Robertson.
In 1891 and 1901 Jane Robertson and Margaret Robertson were at Rose Street Edinburgh. I have found Jane's death in 1904 but would like to know what happened to John and Margaret after that.
Regarding the death of John's wife Betsy in Somerset in 1925, I have now found her death cert, which I had forgotten, and it states that she was wife of John Robertson, Solicitor's Clerk (retired), which suggests he was still alive. I wonder if he also lived in Somerset, so no record of death in Scotland? I can't find anything obvious in English death records as yet.