Answering my last question, the biog articles lead me to surmise it was written some time in (late?) 1969.
Thanks for the latest observation Gerry (are you Gerry Hallom?) - that's a good point. I think the shift comes when the memories become directly personal rather than being generic or common to every worker.
The other issue raised earlier about omitting the chorus between verses 2 and 3 is a peculiar one, especially since Colin does not do so in his own performance, and it doesn't feel right to omit it there.
I also found some unusual phrasing in Colin's own rendition, but by and large I was gratified to discover that I stick very closely to his original words with just a couple of small variations that hadn't previously occurred to me.