My dad did not have a birth certificate for his father, also called Harry Smith, and was unsure where/when he was born. His death certificate gave his age so we knew it was around 1900.
It made it impossible to trace the family tree in that direction although we knew his father was also Harry Smith - confused? I was :)
Dad thought his birthday may have been in April so I trawled the BMD and came up with two possibles so ordered the certificates (no fathers name on BMD index).
It arrived yesterday and suddenly opened a whole line of new family members dating back to 1833.