Hamish,

If you were trying to find your people on the Rootsweb free census page, I'm not suprized that you didn't find anyone. If you look on this page http://home.chilitech.net/~mmjeffery/myweb9/index.htm, it gives the status of the different counties. It looks like little attention has been paid to the Sutherland, Ross & Cormarty and other counties in that area.

Further the rootsweb site covers only earlier censuses (ie. 1841 to 1871). I would imagine you are more likely to find your father's mother and her parents in a later census (ie. 1881 to 1901) -- census have been taken in England and Wales and Scotland every ten years. As you may (or may not know) only 100 year old census are available to the public. So there is nothing available past 1901. You'll have to wait until 2011 to see 1911.

I would suggest, if you aren't opposed to paying a little, going to http://www.scotlandspeople.gov.uk and looking up 1901. My guess is that you will find your grandmother with minimal problem.