signalLegend.removeAllChildren();
signalLegend.direction = "vertical";
for (var i:int = 0; i < numItems - 1; i++) {
    signalLegend.addChild(new LegendItem());
    legendItem = signalLegend.getChildAt(i) as LegendItem;
    legendItem.label = "Title here";
    legendItem.setStyle("fill", theColour);
}

<mx:Legend id="nhLeg"
                       updateComplete="LegendPosition()"
                       direction="vertical"
                       width="80%"/>

private function LegendPosition():void
        {

            nhLeg.direction="vertical";
        }