Well, it's not easy to prove that they're different, but here goes with some examples:

The Holst tune uses the same distinctive phrase (rising from the 3rd of the scale up to the 5th and then down to the tonic)for the beginning of the first, second and fourth lines; the Darke tune does not use this phrase at all, but uses three different bits of melody for those three sections. At the end of the second line, the Holst tune goes back to the tonic chord (is that what you call it? 1,3,5 of the scale)whereas this chord would clash if you played it with the Darke tune. And then Darke uses that flattened seventh in the third line, which doesn't correspond to anything in the Holst tune. And so on.

I agree that in many ways Darke's tunes seems like a badly-remembered reconstruction of Holst's, but what he ends up with is a different (and less good) melody with a different harmonic structure.

Marje