A question! of history!

[desperance]

So I know Dorothy Dunnett had a bit of a Homer's-nodding-again approach to her historical research - today's faux pas is her reference to a thousand metres when first she means a thousand square metres, second I think it's a fairly unlikely figure for the sail of a galley, and third she's a hundred and fifty years too early for the metric system - but even so, I do not think she is confusing a historical figure with a recent Regency romance: so please, somebody tell me, who or what was meant by the Grand Sophy, pre-Heyer? The internet I find very reluctant to disclose this information...