ok - I've dones some more testing and it seems the problem is caused by the invalidly nested form here:
If you remove those form tags then it should work in IE8. You already have a form element around the whole page so you can't nest more form elements.
<li>Enter your Postcode or Town/City</li>
<input type="text" ID="TextBox1" style="width:220px; border:1px solid black;" />
<input type="button" ID="butt" class="postcode-submit" onclick="return butt_onclick()"/>