Thanks for the reply!
I have Margaret's siblings as: Mary Ann b 1858, Elizabeth b 1863, James b 1868 and Agnes Ley b 1874.
Have you found any others?

My problem now is with Margaret Moir Bertie's father JAMES BERTIE. He was 24 when he married in 1856 and I've found a birth on IGI for 1st Aug 1831 which at first I assumed was right as the parents are Helen Skene and James Bertie, but then I couldn't find him in a census.

The census entry for 1841 shows Helen & James Bertie living at the address stated on James and Mary Ann's wedding certificate so it must be the right family, but the James listed there is only aged 6, not 9 or 10. Would they have had 2 sons, both alive, with the same name, or could the age on either IGI, the typed census entry, or marriage certificate be incorrect?
Interestingly, James and Helen's son David is listed as age 7 on the 1841 census yet the only birth I can find on IGI is Jun 1829 so that one's out too.
Does anyone know if Bertie and Skene are both very common names in Montrose - could I be looking at the wrong family on IGI? I found another James Bertie born to Helen Skene and James Bertie in May 1826 according to IGI but that age doesn't fit either. I'm thoroughly confused!