Barry, do you know where Godfrey was stationed with the RNAS/RAF ? If it was India, that would be strong evidence that in one form or another the first stanza about "a troopship leaving Bombay" was originally his. If not, no conclusion could be drawn.

If the song was written in 1917-18 far away from London, it may have been in circulation in a limited way within the RAF between the wars. If the title and chorus contained a four-letter word as well, there would be little chance of its appearing in print during that period, and limited circulation would also make it unlikely that a later memoir could be found mentioning its existence in the '20s or '30s. So one big problem in the song's history may be dealt with by assuming that the song's "folk" currency before 1940 has simply been overstated. It didn't have to be "the unofficial trooping song of the RAF" until 1940 in order to have enjoyed a minor underground existence before that.

Elaborate conjecture is usually a waste of time, but I can't resist in this case. Godfrey, in 1917, could have written a song much like
that copyrighted in 1940, but with the familiar F-word in the title and chorus. It may well have been, then, that either he decided not to pursue publication simply because he thought of the song as something of a topical throwaway and didn't think of replacing the offending word with "Bless." Or, if he did think of it, publishersmay have rejcted the composition because the war was over, India was far away, and there was probably no market for it.

Twenty-odd years later, Keith Prowse published a rewrite and elaboration of what they thought was an unprintable, public domain song. Getting wind of it, Godfrey (and his lawyer) proved to KP's satisfaction that he was the composer and deserving of a share of the royalties. He might have done so with by showing a copy of his original score, with or without the lyrics. For whatever reason, Godfrey agreed that he need not be credited on the sheet music.

For KP to start paying royalties, they must have been convinced that Godfrey was the original author.

Your website, BTW, looks like an excellent resource on neglected pop composer !