Thank you Neale 1961 - The Kirk Session records are a good find and obviously written by the same session clerk as the placename for William is exactly as written on the birth.
Firthhead looks very possible - I've attached an extract of the birth record from Scotland's people.
Still not sure if this is my Margaret Nicholson -although the mother's name of Elizabeth Hunter is compelling. Now I need to look for these people on 1841 Census.
Looby