"Whilst it is highly likely that the Child Ballad was based on the London ballad"
Hardly.
The story of the Keach in the Creel dates back at least as far as the Italian fableux which tells of the apprentice of a renaissance painter employing a trick with a swarm of beetles in order to have his wicked way with his master's daughter - old as the hills.
It was in the 'folk' domain at least in one form or another as early as the fifteenth century.
The broadside version was part of a long line of forms and we have no idea where in the line it came or whether the hacks took it from a singer or storyteller, as is just as likely - even more, as they were not renowned for producing 'deathless verse'.
Jim Carroll