Please, please, please - it's 'unaccompanied' - not 'a capella'!!!
Barbershop quartets do it 'a capella'. Harry Cox, Sam Larner etc., etc. sang unaccompanied.

Not sure I give a toss what Coope, Boyes and Simpson do - I have a strong dislike of singing trad. songs in harmony (but that's just my personal taste).