Thank you Jessden. That was very helpful.
Since first posting this thread, I've confirmed a lot of information about my g-g-grandparents, Thomas Thomson and Elizabeth Glen. I now know Thomas was a Leith blacksmith and he married Elizabeth at St. Cuthbert's on January 1, 1844. Their address at the time of their marriage was Storrars Land, Hillhousefield, Bonnington.
Information from 1841 lists Thomas as a blacksmith journeyman and his address as Smiths Land Fox Lane, South Leith. Unfortunately, I have not yet figured out the names of Thomas's parents. Other Thomsons living with Thomas in 1841 include Janet, a widow aged about 60-65, Alexander, an apprentice engineer aged about 15-20, Jessie (14), and Janet (4).
It seems highly unlikely that 4-year old Janet would be the daughter of the widow Janet, but Thomas, Alexander and Jessie could be the widow Janet's children. I think the widow Janet was born in Liberton.
I've searched extensively through the census and BMD data but can't seem to find any definitive evidence pointing to Thomas's parents. I've emailed the National Library of Scotland to request help in finding blacksmith apprentice records for Leith, which may show more information about Thomas, including the identity of his parents.
I did find an extremely good 1822 map of Leith that shows street names and property ownership, so I can see where places like Bonnington, Hillhousefield, Fox Lane, Elbe Street and Pilrig House are located. I believe Elizabeth Glen worked at Pilrig House.
With your local knowledge, can you suggest any lines of research that could identify Thomas Thomson's parents?