Well Pip, that's where I would be out of my league to comment. It sounds to me like some of Steeleye Span's harmonies, but also reminds me a bit of Welsh choir arrangements. Thirds and fifths is what I thought it might be, but it depends what chords are being used.

Now that we have a sound file to listen to I expect there'll be a rush to provide expert advice, and I can stop floundering about hoping I'm not making a total ass of myself. :-)

Helen