Edward IV and the Tanner of Tamworth

Does anyone know the basis of the legend of Edward IV and the Tanner
of Tamworth? I'm sure I once read a ballad/poem called this. I
vaguely remember that a cow hide is mentioned several times. IIRC
Tamworth Castle in England had a display about the ballad when I
visited it. Didn't William Hastings own Tamworth castle in Edward's
reign in real life? This would explain the setting.

In one of the Bulletins there was a reference to a later (possibly
Tudor or Jacobean) play about "Jane" Shore, in which there is a scene
of a disguised Edward talking to the Tanner, so the story must have
been quite well known.

Is this ballad what lies behind the stories that Edward went around
incognito sometimes? Or are these tales simply another version of
the folkloric/literary motif of the ruler going among his people in
disguise to either cast off his responsibilities and/or find out
what's really happening in his kingdom?

Anyway when was the ballad written? What is the plot? Is it
available anywhere?

Thanks.

Joanne