Hi Jim
It is hard to say if the parents of Alexander Sinclair b 1804 are Alexander and Agnes, as you say he does disappear from Scotland, also a couple of Alexander's b 1804 children are named after his siblings David and George I wonder if the J is James, I would have thought that Alexander would have served an Apprenticeship, if he was a Master Tailor, are there any records for that you could check, for that either in Scotland or England, as he may have left Scotland to do his Apprenticeship
Margp