Many thanks for all your tips. I'll answer some of the questions which have been posed, which may prompt further info.
MonicaL - Susan was shown as "W" in 1851, but no record of Robert's death. I can find no birth record for William Thomson. He doesn't appear in the 1861 census and there is no death record for him.
I think your details of the separate entries in 1861 for Mary and Susan are correct.
CarolW -Mary married Alexander in 1874 and the record shows Robert Tait, a weaver as her father and Susan McLelland as her mother. Only Susan shown in her death certificate, no mention of Robert.
MonicaL - See comment re her death certificate above. I have her death in 1893 in Gavieside reported by her husband Alexander.
Good call on 1871 census - this looks like them and their names are spelled as you have them, on the census! Mary had another child in 1872 recorded as born in Balloney, so that location would seem to tie in with the census.
Once again thank you for all your efforts