Hi Morag

On the 1851 census there is an Isabella b. Dunscore circa 1829 who might fit:
Mains of Waterside, Keir - 833 ED 4 Page 8
Robert Mells or Mills*, 26, farm labourer, b. ? Dumfriesshire
Isabella , 22, b. Dunscore
Sarah Demster, 17, servant, b. Sanquar
It might be worth following her up
Regards
Gadget
Added - I don't see them on the 1861 as yet

* down as Wells on the 1871