Scratch the previous post - It _was_ working perfectly but then I did something, not sure what it was, could have been updates, could have been logging in and out, could have been changing the screen resolution but now no matter what I do the 2nd screen is blanking again in kde3.
this problem is driving me mad - there must be someone out there who knows if it is possible to use mplayer in kde on a second screen without the screen turning off for a few seconds!