Thanks for your comments. I too thought John Robertson at Dunkeld might be a possibility, but the find of John Robertson Whittet makes it unlikely. Also, with Jean and Margaret in Dundee, why would John be living so far away? There is no connection that I can see between William, Andrew and Euphemia Whittet and my Whitton family. Although Jean's father and siblings were called Whittet, all were known as Whitton by 1841. For information, my 'Whittets' were originally from Pitkindie and, earlier, from Kinnaird.
I have looked for John in 1861 under surname Robertson, Whitton or Johnston, but there is nothing that looks right. I suppose he may have been with another family and his surname wrongly transcribed as theirs. Also, we don't even know if he was originally known as John, as his birth registration had no first name!
I am pleased that by returning to this ancestor after a number of years, I have found with your help everything that was missing except for John's whereabouts in 1861. (There is still the question of what happened to Margaret after Jean's death in 1904, but I will start another thread on that !