It's my hypothesis that the Margaret Begg who wed Andrew Lees in Muirkirk on 8 Oct. 1799 was the daughter of Adam Begg and Elisabeth Gibson. The wedding was registered in another parish according to someone's entry in the IGI, they used the spelling "Lorn" but may have meant "Sorn" instead. I'm only aware of one child between them who must have been very wee on account of having been born so prematurely:

Elizabeth Lees, christened Oct 27, 1799 in Muirkirk, Andrew Lees and Margaret Begg parents.

If anyone has any further information regarding this couple I'd like to hear from you.