The Watersons were one of my absolute favourite folk groups during that phase of my youth - and for obvious reasons (if you listen to the above link).
They were three siblings and a cousin (later replaced by a husband) who sang unaccompanied in four part harmony, using an open-throated style that sometimes sounds more like a consort of Crumhorns than modern singing.
Their material was a mixture of pastoral, folk and ritual songs; with a generous measure of revivalist hymns and carols - such as the above.
The basic harmonies are simple enough (although they sometimes spontaneously used harmonies pretty much 'forbidden' in classical music, like open fifths) - but what really adds spice is that they did quite a bit of sliding from note to note (technically portamento) which generates all kinds of transient dissonances and resolutions, especially on the way to cadences.
Their concert I attended in Congresbury, Somerset 1976 or 77 was without doubt one of the premier musical events of my life - in the intimate setting of a small club.