Although I don't recommend the use of an iframe, the first problem that I'm seeing is the use of XHTML self-closing ("/") in the initial iframe tag. That's preventing the closing iframe tag from being used. In fact it's not even making it to the browser. Here's what I'm seeing on your page when I view it on my end:
<h2 class="post-title">Useful Gambling Books and Guides</h2>
<iframe src="http://www.highstakes.co.uk/affiliates/index.php?a=spreadbuddy" frameborder="0" width="100%" scrolling="no" height="2000" />
Notice that the closing iframe tage,
</iframe>, has been truncated. I don't know if fixing this will eliminate the whole problem, but it's a good starting point.