Say “Sir Gawain and the Green Knight” to most people and they usually respond with some vague flicker of recognition. They’ve kind of heard of it, but they don’t know what it is. Was it an Oliver Postgate cartoon, a spin-off series from Noggin the Nog, maybe? Or a Liverpool band from the 1960s. Sir Gawain and the Green Knights — didn’t they once support Gerry and the Pacemakers at the Roundhouse? When you tell them it’s a poem, they say, “Oh, yeah, that’s it”, but they’ve never read it.

It’s different with English-literature graduates. Most students of the canon have had to plough through Sir Gawain at some stage in their studies, either to make a modern translation or to learn (the hard way) the meaning of alliteration.

To those people, remembering it either provokes a ripple of excitement and even a quote from the original text, or revives some facial spasm or nervous reaction brought about by trying to decipher phrases such as: “Thay throngen thender in the thester on thrawen hepes.”

So, just to be clear, Sir Gawain and the Green Knight, as we have come to call it, is an untitled poem written by an unknown author in about 1400. It is 2,530 lines long and written in Middle English. There is only one known copy of the manuscript, and for a long period of history it lay dormant in a private library collection. These days, it lies dormant in the vaults of the British Library, under lock and key, for not only is it a precious object in its own right, it is considered one of the highlights of English literature, “up there with Shakespeare”, so to speak.

JRR Tolkien was one of its early transcribers and translators; in fact, his translation is so faithful, it seems almost older than the original. And it’s not difficult to understand why the poem would appeal to the author of The Lord of the Rings, because, among many other things, Sir Gawain and the Green Knight abounds with the supernatural. The story begins at the siege of Troy — but only to establish a bit of historical and literary street cred — then quickly switches to Camelot, where King Arthur and his knights of the Round Table are enjoying the Christmas festivities. Arthur has made a promise not to touch his supper until something spectacular happens, and right on cue, the doors burst open and into the hall rides a fearsome-looking warrior carrying a great axe. His presence and stature would be terrifying enough, but there’s something else about him that spooks the seated guests: “Amazement seized their minds, no soul had ever seen a knight of such a kind — entirely emerald green.”

The green knight throws down a challenge, daring any man to chop off his head, as long as he can deliver the same blow in return in a year’s time. Sir Gawain, Arthur’s nephew and the youngest member of the Round Table, takes up the wager and performs the beheading. At which the green knight retrieves his head, puts it back on his neck and says to the stunned Gawain: “I’ll see you in 12 months.” Or words to that effect.

So begins one of the strangest and most compelling narratives of all time. The young knight’s heart is effectively put on trial, and his adventures prove a test for other parts of his body, too; on three separate occasions, squirming in his bed sheets, Gawain has to resist the attentions of a very tempting seductress while her husband is out hunting in the field. It’s also one of the first “eco-poems” in the language, dealing exquisitely with the turning of the seasons and with man’s fragile relationship with nature, embodied by the mysterious and elusive green man.

A few years ago, I decided to translate the poem. I did it for all kinds of reasons, some literary, some sentimental, and thought of it as a private indulgence, though it has gone on to be my most successful book, especially in America. I even had a power breakfast with someone from Disney (“We see it as a family film, Simon”), which of course came to nothing. There was also a “political” reason for making the trans­lation. We know from dialect words in the original that Sir Gawain was written by someone living in the south Pennines — Derbyshire, Cheshire, Lancashire, Staffordshire, that kind of area. Not exactly my front garden, but near enough to make me feel I was bringing the poem home.

Geography, or rather landscape, is also at the heart of the Sir Gawain documentary I’m presenting as part of the BBC’s poetry season. Poetry and television don’t always make good partners, but in the case of Sir Gawain, there’s no shortage of things to point the camera at. A year after the incident with the almighty axe (just your average yuletide binge-drinking fracas in this part of the world), Gawain has to gird his loins and set out for the Green Chapel, though he has no idea where or what the Green Chapel is. And it’s this journey that forms the narrative of the film, from start to finish, with me attempting to retrace Sir Gawain’s fading footsteps.

Of course, there’s no more historical evidence to suggest that Camelot existed than there is for Arthur himself, but that hasn’t stopped Arthurians (Trekkies in chain mail) and tourist officers putting pins in the map from Winchester to Carlisle. Few places, however, have embraced the Camelot legend more than Tintagel, in Cornwall. On camera, I read some of the poem in Merlin’s Cave and stride among the castle ruins on the clifftops as poetically as a pair of elasticated over­trousers will allow. I also meet two latter-day knights, Gandalf and Gary, and voluntarily take a punch in the stomach to test the protective properties of a metal breastplate.

Gary: “How was that?”

Poet (swallowing blood): “Well... I felt it.”

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