I found a Jean McLintock b. 1776 Cardross Dunbarton, father James on I.G.I.- this would match with her death in 1856 age 80.
I also remember a while back (I can't remember where I found it now) seeing something about the death of (presumably) Alexander's father: "Andrew Webster, quarryman, Kempock, Gourock died on the shore near Scotia Place Gourock on 11 August 1870 age 96" from the Greenock Advertiser. This makes his birth around 1774- I have found several possible birth records but none of them leap out as being the right one.
Also, a possible marriage for Thomas Watt and Isabella Murdoch, approx 1797 in Mauchline Ayr. both born approx. 1776 in Mauchline.