Though it's true that "Sweet Thames run softly" is a quote from Edmund Spenser, the most probable source of it, for this purpose, would have been from a 1940 and 1947 book of the same name, by Robert Gibbings, author of a series of small scale travel books with his own beautiful woodcut illustrations.
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"I met my love by the gasworks croft fall" is not really a singable line, though Ewan coukd probabaly have got away with it. Better, "by the gasworks croft" - the rhyme scheme doesn't call for a rhyme there anyway. I can't remember how Ewan sang it, but if he did have the line used in Digital Tradition, I'd reckon he was playing games.