Dear Barbara, Thank you so very much for that, it's fantastic! And, as so often is the case, confusing! Possibly my ancestor is in fact William the father of this William. But my descent was indeed through a Margaret H who m a McDowell - but I thought she married Patrick McD of Freugh, not a John McD.
And I can't find the ref to Mckerlie's "History of the Lands and their Owners in Galloway" - either that's a mistake or it's a different edition of the book from mine.
However, the dates fit my original belief that my ancestor was William the son; the son-in-law McDowell came into his property at his father's death in 1692 - so if 'my' William Haltridge died a couple of years later, that would seem perfectly sensible.
Oh the pleasures and pains of genealogy!
Do you have any idea of other sources where I might be able to track down any of this?