The narrator is the author himself, Sir John Betjeman. In his autobiography, Summoned by Bells, written in blank verse (no rhyming), he is narrating the tale of an experience that occurred while a boy at (boarding) preparatory school, called Dragon School, in North Oxford. After a long and varied career that included work in the intelligince field (spying), he was named England's Poet Laureate 1972 after having been knighted as a KB, Knight Bachelor, in 1969.
Betjeman (originally Betjemann) was beloved by a wide audience for his light-hearted though sincere and genuine voice and for his style built on traditional stanzaic and rhyme structures. These made his poems readable, enjoyable, and easy to understand, which is what gave him his wide popularity and audience.
This light-hearted, genuine voice and simplicity is evident in "Percival Mandeville." The story is essentially about a fight that is scheduled to take place between him and Percival. Betjeman is terrified but brave--yet clever enough to find a way out of something he knows can only go badly for him: "I could not box: I greatly dreaded pain." The poem demonstrates the values of manhood bred into the lads at boarding school: courage, ingenuity, cleverness, and gentlemanliness.
Betjeman demonstrates the first qualities when (1) he lies awake in fear while putting on a brave face for the other boys and when (2) he comes up with a clever plan to elude the fight. Percival demonstrates gentlemanliness when (1) he challenges Betjeman to a fight to defend the honor of a student Betjeman seems to have wronged ("Maybe some bullying in the dormitory;") and when (2) he responds to Betjeman's fight escape plan. Betjemann fakes a note to himself claiming his mother is ill. Percival responds by accepting Betjemin's excuse and apology--both taken at Betjeman's word alone--and by putting a comforting arm around his shoulder.
... Silent in the dorm
I cleaned my teeth and clambered into bed.
Thin seemed pyjamas and inadequate
The regulation blankets once so warm.
"What's up?" "Oh, nothing." I expect they knew ...
And, in the morning, cornflakes, bread and tea,
Cook's Farm Eggs and a spoon of marmalade,
Which heralded the North and Hillard hours
Of Latin composition, brought the post.
Breakfast and letters! Then it was a flash
Of hope, escape and inspiration came:
Invent a letter of bad news from home.
I sought out Mandeville. "I say," I said,
"I'm frightfully sorry I can't fight today.
I've just received some rotten news from home:
My mater's very ill." No need for more-
His arm was round my shoulder comforting:
"All right, old chap. Of course I understand."