Jean
Thank you very much for your replies. You have confirmed everything I thought. I found the same family as you in the 1841 and 1851 Census, and by the 1861 Census Fanny had "disappeared". I assumed that she had died between 1851 and 1854, and other than the official Census, as a family they seemed to prefer not to put anything down on paper. I found her "husband's" death certificate in 1866 which has his wife as Fanny Birkhill, so I am not at all surprised that Birkhill seems to be coming up as her true name. The name given on my gggrandad's wedding certificate is Mcalpine (but that is the ONLY place I can find Fanny Mcalpine).
What I had not been able to find was her death certificate, which you say actually has her as Janet Birkhill. This then changes her from Fanny McAlpine to Janet Birkhill, on scotlandspeople arghhhhhhhhh I had been wondering about Mcalpines changing their names, so Birkhill seemed reasonable, but I would NEVER have thought of looking under Janet.
What has happened though is that with only the bear minimum of information which I gave you, you have come up with EXACTLY the same family as me, albeit with the conundrum of Fanny's true name, and a 100% connection to my gggrandad. The main thing which still bothers me is that in the 1851 Census the William in this family is a coalminer, whereas by the time my gggrandad married in 1856 he was a Printer (on the Marriage Certificate). BUT, I DO have a Robert as father, Fanny as mother, William and Francis (all appearing on the Marriage Certificate) and that William came from Bannockburn. The family of Robert Stevenson in the 1841 Census has this same combination of people and living in Bannockburn. So, with the information you have now given me about Fanny/Janet McAlpine/Birkhill, I now have a bit more information to try to firm this family up, and the connection to my gggrandad.
I had been totally blocked, and it is nice to find someone else agreeing with the conclusions I have come to, and.......finding Fanny's Death Certificate.
Thank you so much