This might sound a bit daft but have you tried looking at this from a different angle. It might not help you with your grandfather unless he was the informant on his father's death certificate.
My grandfather, William James Selwood was born, I believe on 29 April 1887 .......
On 14 September 1918 he married Violet Lily Sharpe at Horfield Parish Church in Bristol. His age is shown as 29, which means he was born c.1889. ....
His father's details are shown as being: William James Selwood (deceased), Petty Officer R.N.
His father was dead by 1918
William James Selwood (deceased), Petty Officer R.N. So would looking for his father's death help you find some other clues? He must have been atleast 20 when your grandfather was born. This death looks a possibility particularly as I think Devonport could fit in with his naval connection.
William J Sellwood Estimated Birth Year: abt 1867
Jan-Feb-Mar 1916 Devonport Devon Vol: 5b Page: 482
Age at Death: 49
Laurina