I did wonder if this is an obscure duo called the Cotters. They were a Scottish duo - vocals guitar and mandolin, who made just one album in 1969 for Page One records opening with the Mingulay Boar song. . and then that was it. Both are now deceased. However I am just throwing this one out there, as I can't find any uploads of their recordings.. so it maybe a red herring (or I might just be casting my net in the wrong direction). It doesn't sound like a full harmony group.. and there is no whistle (as in some suggested versions)