Don't know of any connection with Watford. Johnny Todd was a version of the Liverpool skipping song Johnny Sailor - I had a version from my grandmother who came down with her family from Glasgow, at the end of the 19c. They lived on Scotland Rd. and she sang this with her mates for skipping. I recorded it at an EFDSS Festival at the Bluecoat Chambers in 1964.
Fritz Spiegel adapted it as the Z cars theme.