1881 census Mary is 29 born Scotland:
https://www.ancestry.co.uk/interacti...=successSource
and 1891 she is 39 born Scotland:
https://www.ancestry.co.uk/interacti...=successSource
and 1901 she is 49 born Scotland:
https://www.ancestry.co.uk/interacti...3_112_114-0708
On the GRO death index she is Mary Treadwell age 56 (Oct-Dec 1908 Marylebone).
So no middle name of Nicoll on any of these records, nor on the marriage record, and the censuses don't narrow her birthplace down to Forfar. Can I ask how you know she is Mary Nicoll Anderson from Forfar, so we don't cover the same ground all over again, please?