Agree with Neale. Uneducated guess here, but I’d estimate late war years, say 1916-1918.
Another way of looking at it might be to work on their ages and relationships. I think the 2 children would be around 10 and 2.5. The lady on the left and the man in the middle would be the older boy’s parents and the man on the right (your grandfather) would be the father of the younger child, with no mother in the photo, so comforted by his grandmother. (No guarantee it’s a boy but the hair parting makes me think it is.)
Hope this helps.
Anyway, here’s my go at a restoration. Lovely picture. I enjoyed working on it.
Peter