I have been listening to a fair bit of Anne Briggs the last few days, and I have decided that she over-decorated. On this recording (where her own pitch is a bit suspect too) you can hear that the additional singers are a little behind her, possibly because they are having to wait for the decoration to develop before knowing hwere to go themselves (or possibly because of her notoriously personal sense of timing).   

You may well do it better than her.

The instrument is not a mandolin. She played an Irish bouzouki (but it may have been before there was such a thing as an Irish bouzouki) and acccording to some notes I have somewhere she used a drone tuning - it might have been EAEA - and was largely self-taught.