So, I have spent the evening sorting out William Turnbull b. 1857, his mother Elizabeth Jane b.1832 and her parents Willian b 1795 and Margaret Turnbull nee Harding b 1788. I added all William's Turnbull aunts and uncles: Mary b. 1820, Margaret b. 1825, Thomas b. 1825 and James b. 1830. I managed to get a pretty full and convincing set of evidence for each. I am really pleased and grateful Maddys52 that you have provided the keys to unlock the problem of 'Jane' that I highlighted in my first post. Yes, it is clearly Elizabeth Jane with a 1908 death. The missing child, Margaret too - I am convinced you are right in everything you found and suggested.
Given two of the children of William and Sarah Ann have the maiden name of mother as Tipping then that must be more likely than Tiffin as recorded on John James Birth Certificate. My early trawl has not really turned up much particularly useful for a Sarah Ann Tipping though. There is a birth in Q4 1862 Carlisle 10b/422 and an 1881 census of a 19 year old Sarah Ann Tipping lodging at Trinity Buildings in Caldewgate and working as a cotton bobbin winder. Both fit quite well but I lack parents, an 1871 census for her and especially the elusive marriage to William Turnbull which has always been a big objective and a puzzling omission.
With regard to my other puzzle regarding the whereabouts of my John James Turnbull on the 1891 census, I think the one I suggested where there is a 5 year old John James Turnbull, born in Botcherby and living with Smith grandparents is increasingly unlikely to be my John James. I have found another John James Turnbull baptised in Carlisle to a David and Jane Turnbull (no maiden name) on 7/9/1884. I suspect this one is the Smith grandson leaving my John James still missing in the 1891.
Any help or suggestions with these final issues will be greatly appreciated.