In the records of their marriage (at the Roman Catholic St. Mary's Cathedral in Edinburgh), he was described as:
John Philip Smith, plasterer, 24 of 11 St. Andrew Street, Leith
son of John Smith, labourer (decd) and Bridget m/s Munro
His wife Ellen died of pneumonia at the Royal Maternity Hospital Edinburgh in 1922.
There is no record of a Smith child born near that date to those parents. (I looked at dozens of them).
The family's address was 7 Ballantyne Road, Leith.
It would not have been possible to look at all the Smiths born in Edinburgh & Leith between 1905 and 1922 to find the children of this couple (if any). At this time, the birth index for that period did not show the mother's maiden name.
However, when Ellen's sister Annie died in 1942, her death was registered by her brother-in-law, John P. Smith of 7 Ballantyne Road, Leith
As this was 20 years after the death of Ellen, it implied that there had been Smith children.
When I looked at John's own death certificate, he was still living at 7 Ballantyne Road, Leith in 1951.
His death was certified by his son M. Smith of 7 Ballantyne Road, Leith.
It isn't any easier to find the birth certificate of an M. Smith of unknown age.
However, I looked at the Electoral Roll for that address:
In 1926, John Smith was the only adult Smith.
By 1930, John P. Smith, John J. Smith and Matthew P. Smith were listed, and remained listed for some years, dropping out one by one.
The last Smith inhabitant was in 1965, when Matthew P. Smith was the only adult Smith.
This enabled me to trace the birth of Matthew Smith in 1909. He never married, and his death in 1966 was registered by his sister, Mary Watt.
This lead to the marriage certificate of Mary Smith (19) to Robert Watt (22) in 1932, and subsequently to her birth in 1912 and death in 1988.
It was not difficult to trace the births of her 2 sons in 1935 and 1944, and the marriage of one of them in 1956, who had a daughter born in 1956 and a son in 1958.
His daughter married for the second time in 1995, witnessed by 2 other members of the Watt family.
That lead to three addresses and phone numbers from the current Edinburgh Phone Directory.
My client got in touch with his cousins, and found that they had traced the same ancestors back to the same brick wall in 1803.
The only new information that they were able to provide was that Matthew and Mary had had another brother, David, who had not had any children.
However, my client (descended from an uncle of Ellen and Annie) has some new third cousins.