It's in a hollow boulder in the North-east side of Llewdor. Go to the base of Manannan's mountain, then go left one screen to where the bard used to be (you did deal with the bard and got his pitcher, right?) Then go up one screen. The boulder is sitting right next to a tree and has a small hole on the top of it.
Go into your inventory and look at the pitcher to discover it magically fills itself with water if you say the magic word. Use the pitcher on the hole and Gwydion will say the magic word and fill the hole with water. You can then retrieve the missing spell page. :)