Terry, it doesn't sound very likely to me that MacColl wrote the song 'in snatches for different radio shows', as he only made the one on the topic of fishing. MacColl's memories of how this song came into being are clearly somewhat different from Killen's:

[1990:] When I finished writing [this], we sang it to Sam Larner on our next trip up. He was delighted that I knew it for, as he declared, 'I known that song all my life'. [...] A song about fishermen must please fishermen, a song about miners must be convincing to miners, or there is something wrong with it. (MacColl, Journeyman 323)
For The Shoals of Herring I tried out and rejected more than a score of tune models and, in the course of a fortnight, sang hundreds of first-line variants before I found one that pleased me. After that, it was a matter of seeing whether the rest of the tune soared naturally out of that first line or whether it had to be coaxed into the open. (MacColl, Journeyman 365)

Maybe Lou was having people on?