On p. 84 of Lords of Men, there's a sidebar on the Brotherhood of the Black Mask; an English social revolt in the reign of King John against foreign clergy who had (by English standards) been abusing their economic positions.
Costain's bibliography on Wikipedia lists "The Magificent Century" as one of his non-fictional works. Chapters/Indigo has different versions of it in both the history and fiction sections.
Costain did a multi book series on the history of the Plantagenet family. Magnificent Century was also my source on Eustace the Monk and the Maid of Brittany.
I have a copy of The Magnificent Century. I found the section on the apparently vampiric efforts of the Vatican appointees, but nothing on vigilantes opposing them.
I also found an interesting historical character, Willikin of the Weald, who I'd love to work in to something.
But you may look up Enforcers of Magna Carta in the Oxford Dictionary of National Biography, there search for the 'northerners' like William de Mowbray, Richard de Percy and Roger de Montbegon, and you have your members of the Brotherhood - Black Mask or not - right there. These would not have tolerated Italian clergy on their land following the lifting of the interdict 1213, and had ample means to terrorize them, should they ever appear.