Thanks everyone for info. It's been most helpful. Have been trying to follow up the idea of a Fifeshire connection. Have found two families that have an Agnes Thompson born around 1818.
Lilias and William Thompson , Falkland,Fife with Agnes (1817), Anne and William. No Mary.

Also, Christian and William Thompson,St. Andrews and St. Leonard's, Fife, with Agnes (b 1818), Isabella, Christian, Robina Alison and Lawrance. No Mary found.