Worldpay Callback failing

Hi

I've got Zen Cart setup to use RBS Worldpay payments, it was working perfectly and to my knowledge nothing has changed.

I've tried placing an order and it all goes fine, payment gets processed but the order is never updated on the Zen Cart system.

I am getting the following email from RBS everytime someone places an order:

**Our systems have detected that your callback has failed.

This callback failure means we were unable to pass information to your server about the following transaction:

Error reported: Callback to http://www.dustfreehay.co.uk/store/index.php?main_page=wp_callback: NOT OK, recevied HTTP status: 302
Server Reference: mg4imsps1p:callbackFailureEmail-83217:MerchReq-382-8**

I have checked the setup within both Worldpay and Zen Cart and everthing seems fine.

Can anyone point me in the right direction for getting this solved please.