Help me identify a YA novel, please

I read this young adult novel during the eighties in a German translation, it might be quite older.

I am very sure it is set in England. A young boy spends a summer (vacation?) in a small village near the sea and near a moor. He has trouble making friends among the local boys and girls and thus feels very lonely.

But then he hears an old legend that one can ask the moor to send someone who will be your friend to the end of your life. Boy does so, moorboy arrives, the become friends and moorboy helps in defeating/humiliating the local youth.

But moorboy is bound to be boy’s sidekick, but will be free once boy is dead. So he starts plotting his “friend’s” demise.

Two details I remember:

  • a scene about spearfishing flatfishes at the beach with forks tied to sticks.
  • older relative is suspicious of the bew friend and falls ill. When the family visits him in the hospital of the next big city, moorboy sends his regards and a fruitbasket. All the fruit rots once it is carried across the river between village and city.

Does this ring any bell?

No clue, but sounds interesting. Try What’s the Name of That Book?