Thank you for picking up this thread and for that information - I shall certainly go through those lines of your fascinating website carefully and check the details of my own sources. It was an enormous task that you did there to get all the family lines sorted so coherently. These five pages on Roots Chat of rolling-thoughts over time are most interesting.
I am confident that it is the same man, John Anderson, but not entirely sure of his ancestral history. I have his birth pinned down in Paisley in Oct 1793, to Petter/Peter Anderson and Janet Smith, (one day out from the date I had from a Macalister family story, written down by his grandson).
I am also trying to find if any sources of the records of the shipping companies he worked for (including Messrs G&J Burns, David Hutcheson in 1830-1850s).