Replying to what Ottery said,
Interesting. However, there are a multitude of instruments being played in the recording. I hear the guitar and harmonica, and possibly the accordion if I'm hearing right. If Roy is playing the fiddle, harmonica for either he or Ronnie is plausible, but neither of them could possibly play that and another instrument at the same time.

There are sounds from instruments I possibly don't know, and a few instruments the Corries played that operate in a way I don't understand (Combolins), also I often think there are two instruments being played when a Combolin is played. So it's possible that maybe they aren't really playing that many instruments, but I was thinking that maybe someone was playing the other instruments. It wasn't always just the Corries who were playing sometimes too.