Well, it's "sack" as we sing it. It's probably part of the folk process but I'm not sure. Geordies don't use "Sark" (I'm not sure who does since 1066) but McColl's Scottish accent makes "sack" sound a bit like that, so listen for yourself and see. Before they could afford a shirt, colliers did wear sacking to keep them warm. They quite often couldn't afford a shirt either - note, for example, the verse from "Byker Hill"

When first to this pit, I came I had no shirt unto my name Now I've got me 2 or 3 The Walker Pit's done well for me

I personally like what the folk process does to songs anyway and wouldn't want to sing it "correctly"

Cheers! Ian