The key to this one is the census!
In the 1841 census transcription on
http://freecen.rootsweb.com/cgi/search.pl William Robertson, his wife Martha Somerville and daughters Mary and Helen, are listed living in Wilsontown Row, Carnwath. Martha's age is given as 40,
which implies that she was born 1796/1801, so she is about 20 years older than either of your Marthas[/b].
In another household in Carnwath is Martha Somerville, aged 20, so born 1816/21. Others in the same household are John Somerville and Christina Simpson. John Somerville and Christian Simpson were married in Carnwath in June 1831 (if you trust the IGI!) so it may be that Christina is Martha's stepmother.
There are three baptisms of Martha Som(m)ervilles in Carnwath in the index at
www.scotlandspeople.gov.uk1. daughter of William Somerville and Mary Gibson, baptised 7 June 1795
2. daughter of John Somerville and Elisabeth Brice, baptised 9 July 1817
3. daughter of John Sommerville and Elisabeth Bryce, baptised 20 December 1818. (I suspect that the reason Annette found only the last one is that the first two are listed with only one 'm' so would not have come up if she searched with 'mm'.)
I think the overwhelming likelihood is that No 1 married William Robertson, No 2 died in infancy and No 3 married Maurice Kay. Note that William Robertson's first child was named Mary (after Mary Gibson?) and Maurice Kay's first child is Elizabeth Bryce Gibson Kay (Gibson again? Can there be a connection between the two Somerville families?)
George Kay's 1855 birth certificate says that he was Martha's second child, which proves that it was not a case of the same Martha being married twice.