The Boy Who Wasn't There by K M Peyton
A body in the lake - and a murderer on the loose . . .
When thirteen-year-old Arnold sees a dead body floating in a lake, he finds himself in deep trouble. For now someone is trying to kill him too - to wipe out the only witness!
His only chance of escape seems to lie in taking up his new friend Jodie's suggestion to join her on a schools orchestra tour in the Highlands of Scotland. It won't matter that he can't play a note of music. The important thing is that he will be safe. Or will he? For the murderer is a desperate killer who will stop at nothing . . .