At first I thought the one child who is strong enough to overcome the enchantment of the barrow is called Roland after the nephew of Charlemagne, who was the strongest and the bravest knight, but it seems he is called Roland for a fairy story called Childe Rowland. This boy went into fairy land to find his two brothers and sister who were under an enchantment after losing a ball in a church and after the sister went to get the ball but had gone widdershins(counter to the sun).
I was also struck by how much the barrow scene resembled the hobbits' battle with the barrow wights of the Barrow Downs in Lord of the Rings. Most likely, Garner and Tolkien were drawing on the same folk tales. It seems to me the stone circle, with the eighty one uncountable stones, could be any one of stone circles in England but Stonehenge had eighty one Sarsen stones.
The 'Lay of the Starved Fool' that Malebron is using as his own Sibylline prophecy book hearkens to the grail stories because Percival, the original grail knight was also the 'fool'. Some etymologies say Parzival did not mean 'pierced valley' but 'wise fool'. This also leads into tarot because Percival's journey is rather like that of the Fool. The treasures are hidden under a bed of roses, the rose garden being an important location in Medieval song.
The thing that is needed to heal Elidor and Malebron is not a grail but the song of a unicorn. While digging the hole to bury and hide the Treasures, the children dig up and accidentally break a jug with unicorn on it. This is what sets the unicorn free. Roland's sister Helen is needed to capture the unicorn, Findhorn (white horn), but the virgin is usually used to lure the unicorn so it can be killed and Findhorn, although not killed by the children, is killed by the men hunting them. Its dying song brings the light back to Elidor. The different elements of older stories brought together in a new way reminds me of J.R.R. Tolkiens' essay 'On Fairy Stories' and the cauldron of story. You can throw whatever ingredients you want in the 'soup' but of course they have to blend well together and these ones do.
And one last word, if you are not Canadian, you likely have not heard the Unicorn Song but here it is.