"The ballads/legends are pure fiction made up for pageants and suchlike in the 16th/17th centuries,"
Far, far too much of a generalisation Steve, and once again, an uncorroborated speculation only of who made them up or why.
The totally unknown ballad makers, whoever they were, made them up for a whole host of reasons we can only guess at: because the events recorded, real or imagined, touched their lives, because that is how the stories had been passed on to them, because the events depicted, sometimes real, sometimes invented, caught their imaginations enough for the makers and re-makers to have wanted them to survive... a whole host of reasons we can't possibly fathom from this distance in time.
And as for ballad accuracy, these unknown ballad makers have done no more nor less than historians have done down the ages, and presented events and tales as they would like to believe actually happened, or as they would like us to believe happened.
"The previously mentioned Lawrence Price wrote some of them, along with Martin Parker."
As I have pointed out before, we have no way whatever of knowing whether any ballad existed in oral form prior to the above mentioned getting their hands on it, or if we have, it is yet to be demonstrated.
We do have examples in places where a healthy living tradition survived (19th/early 20th century Ireland, for instance - try looking up the events surrounding Farmer Michael Hayes sometime), of ballads being made anonymously as political weapons, as rallying calls to action, as a gesture of triumphalism, or despair, or anger or simply to record an event that would otherwise have been forgotten - far, far more reliable a guide than unproveable speculation on 16th and 17th century creations about which, as I said, we can only hazard a guess.
That these pieces get changed and adapted as they are passed on orally is inevitable and is as important a part of their role in our history and culture as was their creation in the first place. Dismiss this fact and you dismiss any idea that 'ordinary people' (if there ever was such an animal) played any part in the recording of their/our history.
If any of us actually knew who wrote the ballads and why, perhaps we'd be legends in our own right, and maybe somebody would have made up ballads about us!!   
Jim Carroll