OnClick events - using Scaleform UI with Cryengine 3.5.4

I've created a UI in Flash, set up an xml file for it and designed a flowgraph to ensure it displays when I launch. However, clicking on the buttons doesn't appear to do anything, despite me writing a click event in AS, linking to it in the xml, and connecting the function to a level change in the flowgraph.

XML:

<UIElements name="zlogicGUI2">

<UIElement name="GuiV2" render_lockless="1">

<Gfx file="GuiV2.gfx" layer="0">
<Constraints>
<Align mode="dynamic" valign="center" halign="center" scale="0" max="0" />
</Constraints>
</Gfx>

<events>
<event name="OnClick" ExternalInterface="ButtonClickPressed" desc="" />
</events>

</UIElement>

</UIElements>

ActionScript 3 (created in Flash CC)

button_1.addEventListener(MouseEvent.CLICK, fl_MouseClickHandler);

function fl_MouseClickHandler(event:MouseEvent):void
{
    ExternalInterface("ButtonClickPressed");
}