I just found this thread and it saved me a lot of time. Thanks guys. Just a note to anyone else on here, it took me a while to figure where to put these bits of script. I've now sussed that by putting them within the action script for each screen. With a transitions on the screen, this gives me this:

on (reveal) {

// Transition behavior
if (!eventObj.target._isSlide || (eventObj.target._isSlide && eventObj.target.currentSlide)) {
mx.transitions.TransitionManager.start (eventObj.target,
{type:mx.transitions.Blinds,
direction:0, duration:2, easing:mx.transitions.easing.None.easeNone,
numStrips:50, dimension:0});

eventObj.target.__transitionManager._triggerEvent = eventObj.type;
}
// End Transition behavior
stop();
function goNext(){
_parent.currentSlide.gotoNextSlide();
clearInterval(timer);
}
var timer = setInterval(goNext, 4000);
}

Now my only problem to figure out is how to keep the image from screen 1 as the background to the transition into screen 2!

Thanks again guys.