Thanks for all your help

I did manage today to get one answer from my first post.
I called Inverclyde Council and asked about the lair at Port Glasgow cemetery and then were brilliant giving me the information I needed so I now know that the Charles Williamson buried there was infact the brother of William James (child of Charles Williamson and Margaret Harper).
He had died in Glasgow and the death registered there although he was living with his brother at the time in Port Glasgow and the person listed on the death cert was his younger brother George Williamson c1900 and at the time (1957) he was still living in Waterside Londonderry although I could not read the street address on the cert.
Finding the Irish side of the tree very hard as I can find nothing else about the family on-line but have added these details in the hope someone might come across the post at some point - so strange that I can find no one else researching the same family.