Hi cazpro,
When you get the marriage certificate for Mary and James it will show their ages. You will then have a rough idea of when James was born and so will be able to look for his birth. It will also show their occupations and those of their fathers.
Do you have your dad's birth certificate? That will show the names of both parents, where and when he was born, father's occupation and who registered him.
Could Harry have been related to Mary or James? To have been on the electoral roll in 1948-1950 Harry would have to be over 21.
Hope this gives you some help,
Nanny Jan