Thanks, Mick and Michael.
I've now had time to look at and compare all of the extant broadsides and the result is very interesting. It's not often you can trace the geographical spread of a ballad with much accuracy but the variations in this one show earliest as posted above and printed in Dorset (not necessarily the first) then to Hampshire(Young Johnson) with Oxlade of Portsea, then to London with Pitts and followers, spreading north to Birmingham and Manchester and another thread to Chesterfield, York, Newcastle and Cumbria and in the latter 4 he becomes Young Johnston to further confuse with the Child ballad. All printings possibly before 1820.
I'm still convinced it's based on at least one actual event.

Mick, yes the mss give Rempton. I think a quick check of local names should sort this one.