Need to get Paypal standard payments working with an Opencart website please.

General information for the website: http://decoranddesignuk.com/
Kind of development: Customization of existing website
Description of requirements/features: I have been trying to get Paypal standard payments working with an Opencart website.

The site itself (decoranddesignuk.com) performs fine when processing the order and it takes users to Paypal for payment and even returns them afterwards with a success message and order number, but no orders are showing on the site admin or on the user's transaction history and stock is not being deducted.

I have tried setting up IPNs on Paypal but with no discernible difference. I have checked the forums and allI have found is references to an .htaccess issue, but I am not using .htpasswd, so it doesn't seem to simply be that.

This is my first ecommerce site so it could be I'm being thick and doing something obvious wrong - hence the need for some expert help! Thanks.
Payment methods: Paypal
Specific technologies required: Opencart/Paypal integration
Extra notes: