Ok, cheers. I'll do my best here.
All I'm doing is to click on an item (the only item in the shop that I've put any products on (in this case Green & Blacks Dark cherry chocolate)). I've added 3 items to it and whenever I enter 1, 2 or 3 into the shopping basket and then try checkout I keep getting the following message.
Whoops! Sorry, but you are not allowed to perform the action requested.

You are still logged in to your account and may continue shopping. Please choose a destination from a menu.

So to answer your other questions
1.https://www.rentacart.co.uk/~menuorg..._page=time_out
2. Zencart version 1.3.7; PHP version 4.4.4; MySQL 4.1.22-standard; Apache 1.3.37(?)
3. Hosting comapny - rentacart.org
4. I bought an SSL - think was 128bit encryption
5. No addons that I know of
6. No, its never been upgraded.
7. Template is AppleZen
8. The Zen cart was already loaded into the template

Hope all this helps and thanks