Hopefully I am posting this in the right section...
As mentioned above no banner is displayed, if I view the source of the iframe that adsense creates the source only contains:
I think that there is something in my markup that is preventing the Adsense code from working properly. Problem is I have no idea what this could be. So the question is, could anybody point me in the direction of things to check.
If it helps at all the url for the site is therevcounter.com/forum
The iframe src attribute is referencing a URL that is throwing a 403 error. Which means you are not allowed to access the content. Perhaps an authentication configuration issue.
Try comparing the code you are using with the code that Adsense says that you should be using.