Like Steve Parkes, I saw the BBC tv documentary 'Shoals of Herring' in the mid1970s. In fact, I believe we watched it together. It was produced by McColl's pal Philip Donellan who died in Ireland a couple of years ago. After his death the national Film Theatre ran a season of his work, so they might have 'Shoals of Herring'.
There was, indeed, a new closing to the song written for that film, which I believe was:

With our nets and gear we're faring,
To reap a harvest that we do not sow,
But if we don't seed
There will come our hour of need,
When we shall find no more the shoals of herring.