A Kestrel for a Knave was a book by Barry Hines published in 1968. It was adapted for the Ken Loach film Kes.
The protagonist, Billy Casper, was played by actor David Bradley who later had to adopt the stage name Dai Bradley, because there was already an EQUITY member, the RSC actor David Bradley. You may know the latter from many a TV drama, as unintelligible Arthur Webley in Hot Fuzz, as Filch in the Harry Potter films, as the first Doctor in Doctor Who and as William Hartnell in An Adventure in Space and Time, and Walder Frey from Game of Thrones, etc…I met him once, I mentioned it before.
As for Dai Bradley, he was also in the film Zulu Dawn, a couple of other films and various TV parts.
When the book was reprinted after Kes the film, they used the infamous scene of Billy sticking two fingers up for the cover…I grew up with this book…I was infamous by proxy as a child.