There are three deaths for Martha Somervilles in the OPR burial register for Carnwath.
1st one died 2nd March 1847, abode given as Forth, aged 50 years, no cause of death.
2nd one died 6th April 1847, also in Forth village, aged 25 years, no cause of death stated.
3rd one died 14th February 1854 at Braehead, aged 42 years, cause of death given as consumption.
Looks like the 1st one could be the one you want. No record for William Robertson, sorry.
Lodger.