Thanks for responding,
not sure I entirely follow you, I'm being told I need to:
"surround your <canvas> with
<div id="mygame" style="position:absolute;height:100%;width:100%;top:0;left:0"></div>"
but without the canvas tag in the exported html, and just using the 'div' id's it shows the ad, but above the game rather than overlaying the top of it