Magento CE 1.7.0.2 Code issue. Ajax call from custom product view page returns error.

Ajax call from custom product view page returns error, but browsing the ajax page directly with $_GET data in url works?

I will attach all relevant full copies of files.

Basically when the finalMasterChange function runs on the view_custom_drawer.phtml page I recieve the following error:

Fatal Error: Call to undefined method:
Mage_Catalog_Model_Product_Type_Simple::getConfigurableAttributesAsArray() in customajaxmain.phtml on line 12 which is this line:
$productAttributeOptions = $_product->getTypeInstance(true)->getConfigurableAttributesAsArray($_product);

For some reason this seems to be returning the product type as simple even though the product id used is for a configurable product?

Also if you run the custom page directly including the $_GET data in the URL there is no error received and the page returns the desired value:

http://kitchendoors.newportwebdesign.co.uk/customajaxmain/ajax/index/pricetype/price/height/150/width/300/prodid/9038

So why does it fail when the same page is run using an ajax call from the custom product view page?

I can send copies of all files on request.

If you think you can solve this problem please bid, for further information or a full walkthrough of the issue. I can use teamviewer to walk through the problem if needed.