I've read The Man Born To Be King at least half a dozen times since my dad introduced me to it in high school, but it wasn't until this weekend that I was really struck by the legal procedures driving the action. The tenth play, "The Princes of This World", focuses entirely on Jesus' trials before the Sanhedrin, Pilate and Herod. I knew about the trials, of course; they are a part of the events leading up to the crucifixion, but my brain always glossed over why it all happened as it did -- "He suffered under Pontius Pilate" generally being enough. Suddenly, though, it was clear: we're looking at the interplay between Jewish ecclesiastical law and secular Roman law. Two different legal systems had to be satisfied.
I'm relying on Sayers's own always-excellent research for this, but for those of you who find this sort of thing interesting too, here's a bit of detail.
There were two ways to bring evidence against someone under Jewish law at the time: 1) the testimony of two or more witnesses and 2) the Oath of Testimony, which the accused had to answer himself. The former was by far the more standard approach. For the testimony to be considered valid, however, "Jewish law required the exact verbal agreement of at least two witnesses".  When the Sanhedrin was unable to get two witnesses to exactly agree on the statements needed for conviction, Caiaphas turned to the rarer Oath of Testimony.
1st Elder: [B]etween witnesses who all say different things and a prisoner who says nothing, we shall be here until tomorrow.Jesus, who had remained silent to that point, was essentially forced to implicate himself. The next part was always the clearest to me: a death sentence ordered under Jewish law had to be ratified under Roman law.
Caiaphas: I will interrogate the man myself.
Nicodemus: My lord, that is barely legal.
Caiaphas: Barely legal, Brother Nicodemus, is still legal. He shall answer under the Oath of Testimony. If he still refuses to speak he is self-condemned. 
Caiaphas: By our law he has already been convicted and condemned to death. But by Roman law we are denied authority to execute the sentence. Which leads us to Pilate. To Pilate, most of the charges Jesus was facing would not have seemed like proper violations because they had no real equivalent under Roman law.
Pilate: Ye gods! What's all this stuff?--Blasphemy, sabbath-breaking, witchcraft, law of Moses--pages of it. I suppose they know what it's all about. It wasn't until the charge of treason is raised that Pilate sees anything to consider truly dangerous or requiring his signature.
Pilate: By what offence [sic] has he incurred the death-penalty?The treason charge was thrown in to ensure Pilate's conviction under Roman law too.  The subsequent back-and-forth between Pilate, Herod and the religious leaders shows the Roman reluctance to deal with a foreign legal system.
Caiaphas: He pretends to be the Messiah.
Pilate: What does that mean?
Caiaphas: It amounts to a claim to be king of all Israel.
Pilate: There is nothing about that in your court proceedings. I understood he was condemned for blasphemy.
Caiaphas: To us, such a claim is blasphemy, but in Roman eyes it is presumably treason.
Pilate: I see. This is a new charge: treason to Rome. 
The events which are recounted in the Bible and which are further explained in The Man Born To Be King are an old example of the juxtaposition of two different legal systems, one a preexisting religious system and the other a secular system laid atop the native laws. The two systems are concerned with enforcing different laws and violations, and while the interplay between them may sometimes be clumsy, the story of Christ's trials and conviction shows how two different systems can be melded together to achieve one end.
Part of Sayers's reason for insisting on writing these plays in modern English was to strip away the remoteness and rigid sanctity through which most people viewed Jesus' story. "[T]he language about him and the worship offered to him seem utterly remote from the speech of men today and from their pressing needs," the BBC's director of religious broadcasting wrote at the time.  Sayers succeeded in this remarkably well -- the characters truly do come to life again, particularly for people who are just used to the text of their preferred Biblical translation. And more than just the characters are illuminated in a new way -- the events are as well, allowing us to see the facts like ancient legal procedures clearly as well.
 Sayers, Dorothy L. The Man Born To Be King (London: Victor Gollancz Ltd, 1943).
 Ibid, at 261 (emphasis provided).
 Ibid, at 274.
 Ibid, at 278.
 Ibid, at 277.
 Ibid, at 278.
 Ibid, at 270.
 Ibid, at 12.