Hi Deb
I see her parents William & Jeannie STEWART and 3 children you mention, in Motherwell in 1901.
In 1891, William and Jeannie STEWART and their 2 children are at Braeside Cottage, Kirk Brae in Cambusnethan at the home of Jeanni's parents, the HEGGIE's.
Baby Agnes is just 9 months old.
On Scotlands People:
There is no birth male or female registered between 1895 and 1901 and up to 1911 with the forname *len*x and surname St*rt in whole of Scotland, let alone Lanarkshire. Nor is there anyone with the forename "Nana".
There is a single birth of a female in Lanarkshire with the surname St*rt and forname *len*x between 1881 and 1901. It was in 1890 and she was: Agnes Lennox Stewart, birth registered in Cambusnethan.
This birth registration looks like baby Agnes from the 1891 Census, despite the 1891 and 1901 both saying Agnes was born in Wishaw.
Now, it is not unknown for Agnes to be pet-named to Nancy or Nan - I have seen instances of both (and one of my gtxgrans was Agnes, known as "Nancy"). In Scotland, Ann and Agnes can also be interchanged, especially in earlier records. Nan, is also seen as a pet-form for Ann (I have one of those in my tree, too!) . In addition, of course, "Nana" is a pet-form of "Grandmother".
Are you sure your Gran is not actually Agnes? When your grandmother married, what name was it under (who did she marry?) and how old was she stated to be on the marriage?
Cheers
AMBLY