The worm in the well - William Mayne

Robin and Meric are the sons of neighbouring squires. One day, on a fishing trip into the woods, Meric disobeys the orders of the witch, Granny Shaftoe, and shortly after, disappears ...

Robin misses his friend, but occupies himself telling stories to his son, Alan, and the foundling, Margaret. Before long, these two are old enough to go fishing at the well deep in the woods. They meet Granny Shaftoe along the way, and she tells Alan that he must keep whatever he catches. But when Alan reels in his line, an appalling monster emerges, begging not to be thrown back. Alan, horrified, disregards the monster's pleas, and lets go - and turns to find that Margaret, too, has disappeared.

It is soon time for Alan to fight for his king. On his return years later, he is appalled to find that a strange, man-eating worm has laid his town to waste . . .