"Northwest Passage" An a cappella song, it features Rogers alone singing the verses, with several guest vocalists harmonizing with him in the chorus. From Wikipedia

Peter Bellamy was a unique singer - no-one else sounded like him. And he was a great influence on Stan Rogers' band too.

Night Drive: Travels with My Brother; Here, Garnet introduces us to a full cast of characters, including bassists David Eadie, Dave Woodhead, the unforgettable but short-termed bassist "Klag" and Jim Morison; a number of unnamed and pseudonymed "managers," with Jim Fleming among those named; and folk artists such as John Allan Cameron and Peter Bellamy who influenced the band so much.

I wonder if Garnet or another member of the chorus will tell us who it was.