Montfort The Revoutionary 1253 to 1260 (Montfort The Founder of Parliament series Book 3)

The Revolutionary.
Restored to his titles Earl of Leicester, Steward of England, Simon, as permanent ambassador to France, is living contentedly in Paris when Prince Edward commits an atrocity, and the Earl must save him and his friends from Llewellyn.
In England again, unbidden, Simon becomes entangled in the barons' revolt against King Henry's misrule. He rescues the Provisions of Oxford, the constitution of the new, elective government -- and modern democracy is born -- at risk of his life.