[update]
well, only today i finally managed to solve this one!
trashing snapz pro's preferences didnae work. neither did opening the preferences in 'property list editor' and changing the co-ordinates of the palette to put it back onscreen - when i invoked snapz pro, the palette appeared in the centre of the screen for a fraction of a second before sliding like the proverbial "s.h.i.t off a shovel" off to one side and back to lurking halfway offscreen again!
in the end i altered snapz pro's prefs again, set them to "read only" and invoked snapz pro. hurray! - my palette was back where it belonged, in the middle of the screen. but what a feckin palaver! - just to recover a palette that had partially strayed offscreen. even then, snapz pro somehow managed to change the settings for the prefs file back, so it was no longer "read only"