The books are adaapted in the chronological order of the stories, rather than the published order.
The first four novels were broadcast as complete programmes, for the repeats they were split into 3, 4, or 5 parts.
These adaptations of the Narnia stories were done by "Focus On The Family" and are available for any radio station to use; I've put the BBC7 broadcast dates as the original ones because for most people that'll be the station they were first heard on.
1 - The Magicians Nephew