It looks like something Jake and Dinos Chapman might do if they turned their hand to the creative defacement of illuminated manuscripts: the Viz-style gargoyle just to the left of the plainchant notation on page 77 of the nearly 700-year-old Bangor Pontifical, one of the treasures of the Welsh medieval world. On a road trip through the country to create a Welsh Christmas for this Saturday's Music Matters, I saw, handled and turned the pages of this book at Bangor University. The Pontifical is one of the fountainheads of Welsh music history because nearly two-thirds of the manuscript contains musical notation as part of the services the bishop would conduct – including a handful of chants, such as In civitate domini, that are found nowhere else in the world.

And yet in the middle of this beautiful Latin hand – the meticulous gold-leaf decorations and square noteheads of the neumes looking as vivid as they must have done in the early 14th century, and every vellum surface of the book seeming to speak across the centuries – there's a cartoonish scribble of what looks like an unshaven 21st-century bloke with curly hair, a big nose, bejewelled beard, flat cap and shades. Except that it's not. According to Sally Harper, leader of the Bangor Pontifical Project (have a look at more images from the book here), this is an original piece of medieval satire the scurrilous scribe included beside the chant, which would have been sung to consecrate a church bell. It's mix of the sacred and the profane is touching, funny and strange, and makes this manuscript and its music vividly part of the modern world. It makes me want to learn how to read neumes properly, too, but thankfully instead of my untrained warblings you'll be able to hear a real musical medievalist bring this chant to life on Radio 3 on Saturday.