Hello Rob. It's always nice to find another Horsburgh "cousin"!
Can I just point out that the surname you give as Fleet is actually Flett. It's a common name in the north-east of Scotland and the Northern Isles. Many years ago I helped a lady in Cellardyke to research her Horsburgh and Flett roots. I think she must have been a cousin of your mother. Maybe you know who I mean. She told me that after Mary Flett had been in Pittenweem for some time, married to Robert Horsburgh, skipper of the "Garland", she wrote to her parents saying what a great place Pittenweem was so they packed all their possessions into their fishing boat, sailed round the coast to Fife and settled in Pittenweem. There are still Fletts in the area. Sorting out the family-tree in Findochty/Portknockie isn't for the faint-hearted, though, as there are so many Fletts up there.
You have traced your Horsburgh family-tree back to George Horsburgh, shoemaker from Elie, who married Elspeth Stevenson in St. Monans in 1750. George seems to have been the George Horsburgh born in Elie in 1720 to William Horsburgh, shoemaker, and Mary Bickerton. The famous James Horsburgh FRS, chief "hydrographer" or seachart-maker to the East India Company, also seems to have come from this family. There is a plaque to his memory in Elie church. A rock named after Horsburgh stands in the sea near the entrance to Singapore harbour.
George Horsburgh and Elspeth Stevenson were the parents of George Horsburgh who married Janet Thomson of Pittenweem in 1781. Janet was born in 1754 to John Thomson and Isobel Morrice or Morris (m.1745, Pittenweem).
I hope this is of some use to you.
Harry