I think the answer may lie with one of the rebels mentioned in his poem "A Song For England's Rebels" - Robert Ket/Kett, leader of the 1549 Norfolk rebellion. At a place called Moushold Hill there was a large oak-tree, which Ket called the 'Tree of Reformation' where he and his men would debate and hold courts of justice. Some of the rebels were later executed on this tree. And near Wymondham is an oak referred to as "Kett's Oak". Do a web search you'll find lots of references to this.

Mick