It is odd, isn't it, the way that genealogy makes the world seem very big and very small at the same time.
I did manage to get information from the 1861 and 1871 census that gives me what I wanted. The 1861 says 'Forfar, Forfar' and 1871 'Forfarshire, Dundee' - which conflicts a little with the 1851s 'England'!
Unfortunately, I can't find birth information for William Buist, although I did find information for 3 males who I am guessing were his brothers (one of whom is named James), or for the man I assume is his father, Richard Buist. I also can't find birth information for your Susana, or for any children of James Buist in Dundee City or Angus. I guess I will keep hunting for a while, but it looks pretty hopeless at the moment.
Thanks, again, for your good advice.