Hi
I think that you may have to send for the, birth certificate, of the oldest child. to check if you have the correct parents, this should give you an address. then check the Electoral Registers for them, other that that, you could put your story in one of the local papers, to if anything come of that.
Also check to see if any of the children, married and had children, if they did there is always Facebook to check.
Margp