Ah, not even a lot of digging needed. Here: en.wikipedia.org/wiki/Duke_of_Rutland
So, the platagenets had the title first, then it was re-created for Thomas Manners, whose mother was Richard Plantagenet's granddaughter.
mad - that could well be, couldn't it?! Yes.
I've just remembered another good historical novel that's mostly about normal people but with a context to do with this stuff - A Load of Unicorn by Cynthia Harnett. I love her work. It's beautifully written, because it observes very early on that it is set just after the Wars of the Roses 'ended', and everything was peaceful ... although, there was a young Lancastrian prince, Henry Tudor, over in Brittany, nobody thought that mattered ...
I love it because I imagine that is how people would indeed have thought - and as she logically points out, people must have been plotted in undercover ways for Henry's invasion to be so successful, with people thinking it was all peaceful.