Added the following to the Tumblr page:
<body class="{block:IfDarkColourScheme}dark{/block:IfDarkColourScheme}" onload="iframeResizePipe()">
<!-- Added by dfcarney, along witht the iframeResizePipe() call above -->
<iframe id="helpframe" src='' height='0' width='0' frameborder='0'></iframe>
<script type="text/javascript">
function iframeResizePipe()
{
// What's the page height?
//var height = document.body.scrollHeight;
var height = $(document.body).height();
// Going to 'pipe' the data to the parent through the helpframe..
var pipe = document.getElementById('helpframe');
// Cachebuster a precaution here to stop browser caching interfering
pipe.src = 'http://localhost:4000/helper.html?height='+height+'&cacheb='+Math.random();
}
</script>
<!-- END added by dfcarney -->