Well, John was married twice! The IGI has member-submitted entries for him but they can't be relied on, you need to check these with the parish registers John Strangeways DONALDSON
Birth: 10 Dec 1794 Alnwick, Northumberland
Christening: 23 Oct 1795 Alnwick, Northumberland
Father: Capt Thomas Donaldson
Mother: Mary
SELBY - so that's where that part of his name came from and obviously he added it later
Marriages Mary Dorothy Grey 1 Jan 1823
Some pages of the bishops' transcripts are signed by the Curate, and attested by Jno. S. Donaldson, churchwarden of Ancroft
(starting on page 25 of the transcripts) 9th August 1816 John Strangeways DONALDSON, bachelor, married Elizabeth HENDERSON, spinster
baptism 14th April 1818 Margery Strangeways daughter of John Strangeways and Elizabeth DONALDSON Cheswick House, Esquire
baptism 2nd July 1819 Margaret daughter of John Strangeways and Elizabeth DONALDSON, Cheswick House, Esquire
no record that I can see of Elizabeth's death
2nd marriage must have taken place elsewhere to Mary Dorothy GREY
baptism 3rd November 1823 Eliza Mary Selby daughter of John Strangeways and Mary Dorothy DONALDSON, Cheswick House, Esquire
death 19 August 1824 for a Margaret DONALDSON of Cheswick aged 74, probably John's mother?
Will leave you to look for all the others, but there is a Thomas 1824, Henry 1825 and in 1830 John Strangeways DONALDSON, so I think he is probably the John Selby that I found with Robert Selby on the 1841 census
Hope this has given you what you need, enjoy searching the transcripts, hopefully you will find his birth death and 2nd marriage too
Kind regards
Barbara
