Thanks for the various replies.
The son's name was William James Powell, who has no birth record in the UK. He was born between 1902 and 1906. From his death record (1953 in Croydon, Surrey) age 50, earliest could be 1902. But, his marriage record (1948 in Camberwell, London) age 42, suggests birth about 1906.
The boy arriving in 1907, aged 10, could have been an older brother, I suppose.
Eliza Johnson was my gt-grandmother's sister, who last appears on the 1891 Census, as a nurse, working for a doctor in Bristol. I think that she was living with her father's youngest sister, who lived in Bedminster, which might account for Bedminster as her birthplace. Her younger brother, James, went to South Africa with the Somerset Light Infantry during the Boer War. I think that Eliza probably went with him.
The William James Powell, concerned here, does not appear on any UK records before his marriage in 1948 to Edna Holt.
I suppose he might have served in the British Forces during WW2, but have no way of exploring this. His marriage record says that he was an engineer servicing launderette machines, which might suggest REME or something similar.
Gordon