. . . well, well, well! i did treat meself to quite a number of vaughan williams' symphonies last night . . . but they mostly sounded so VERY similar to me ears! though i'm absolutely convinced now that this actually IS an excerpt of one of his symphonies (there are countless passages in this vein all over the place!), i believe it takes a real vaughan williams expert to really spot this exact 45 second snippet somewhere. i pretty much gave it up now - too confusing for me.
(i only think i can rule out the "london symphony" - doesn't sound like it.)
anyway, i'm still open for the correct answer, if anybody might be able solve this riddle . . . !
(what about you, mr scarrott?)