Thank you for everyone who has taken the time to reply to this post.
Dexile, thank you for taking time to photograph the headstone, it is very much appreciated.
I had come across the headstone previously on findagrave website, but his year of death had been mistranscribed as 1805, rather than 1803.
Since my post, I have learned so much more information to 100% verify that this is indeed the correct lair.
shanghaipanda, thank you for taking the time to transcribe the headstone. His wife was indeed Jean (Redpath), which has been confirmed from the OPR burial records, their wills and obituaries.
Little Nell, thank you for the links, again its much appreciated and very helpful. The maps will give me a better idea of where they lived. I have been able to trace where they lived from various OPR burial records, wills, and of course the P.O. directories which have been invaluable in confirming each record.
Forfarian, the OPR burial records for Leith have been a great source, and well kept compared to many other parishes.
I have found so much more information on my 7x great grandfather, John Thomson, charting his impressive naval career, which ended with him as master and lieutenant on Indefatigable during the French Revolution after the battle of Droits de l'Homme in 1797. He retired from the Navy soon after this, having been injured in that battle.
Of my 6x great grandfather, Lawrence Skeen/Skene, I have learned that his ship, The Skeen of Leith, was chartered by Gregor McGregor to transport people to Poyais. Poyais was sold as being the land of milk and honey, and was of course a figment of McGregor's imagination. Fascinating reading for sure, as I had never heard of Poyais, but its had me gripped.
I have also found old court documents and newspaper articles relating to John Thomson's son, John junior, Lawrence Skeen, and James Bell Dow (my 5x great grandfather - who was their solicitor for the court cases). So that was a real treat to find them all on one official document together, and was the icing on the cake for me.
Once again, thank you everyone.