The Master of Maryknoll

I have hidden major spoilers to aid my memory. Read them at your peril.

Musical Boy runs away from his uncle's home and joins up with 2 children to go on adventures. The baddy wants to steal and sell the boy's valuable violin. He and his henchman butler kidnap him, and when he escapes, pursue him across the Shropshire countryside   
Children's story, scarier than a Blyton, but scary situations resolved pretty quickly.