It was was common for someone a stepfather as as their father on marriage, sometimes a grandfather or uncle, even a fictitious person to conceal illegitimacy.
I do know a SINSBURY from the area, but have been unable to link him to your family, although I suspect there is a connection if it was searched back.
Thought you may be interested in these pictures of where the family subsequently lived, Pound Lane, and the rear of Spicer Street, both lost in the development of the by-pass, although a tiny fraction of Pound Lane remains at the start of the by-pass.
Jebber