Yup, I've heard that story too, and it may in fact be true. On the other hand, I think there's some circumstantial evidence which points to it being by Mannock: If you compare it to this other ditty written by Mannock (this one relating to
John Tudhope shooting down
Hans Waldhausen's Albatros), there is a bit of structural similarity, notably the use of parenthesis in the first section and the personal comment at the end:
There was a little Hun
Who ventured O'er the lines,
(A risky thing to do,
in these riskiest of times.)
Said the airman on his tail,
in a voice so sweet and calm,
'another thousand revs wouldn't do you any harm'.
The structural similarities could be mere coincidence of course, and one of them compares somewhat to the Lewis Carroll original, which does tend to explain that. And it could even be a case of Mannock simply mimicking the Jabberwocky parody's structure because he liked it, if the story of William Bond having written it is true. It would be interesting to find out who genuinely is the author, but unfortunately, we may never know that for sure. Another rather obvious problem with Peter Hart's version, which points to poor research, is that he says Jabberwocky was by Lear, which of course it wasn't, and itself a parody of Edward Lear's nonsense style, but not knowing who wrote Jabberwocky doesn't mean his claim is not correct however.
Al