Thanks for checking that, mjmaltby. 🙂 If that’s the case, then something very deep in your site is going on. We know that that configuration should work, so I suspect the behavior is related to running on MS/IIS7 (as your web server). That setup, alas, is prone to being a difficult platform.
We can see the post data (the product ID, quantity etc.) is correctly being submitted. My guess is, for whatever reason, that it’s not actually being received or passed into the web app. The fault may lie in a firewall setting or other IIS web.config issue.
That’s a bit of a stab in the dark but I think there’s a good chance something like that is at work.
I’m sorry we couldn’t be of more assistance. Please do let us know when you’ve figured it out! 🙂