The Secret of Rumbling Churn

Written in 1960, this is a typical British kids' book of that era. A group of Boy Scouts (the Cuckoo patrol) go camping in an out-of-the-way corner of England, and run into a mystery. Fairly well written, with some good details on spelunking. It was popular in its day, and there should be lots of copies around. A movie based loosely on the series starred Freddie and the Dreamers.