Using 1.3.9h - new install. Accessing via Safari.

I have products with attributes. I can determine the url for each combination of interest by adding to the cart and copying the url that it has in the cart. All good.

Now I need to provide links from the outside world to these products with the relevant attributes set.

The links work perfectly from within the cart, but if I enter that same url directly into my browser, it takes me to the product, but without any attributes set.

EXCEPT: it seems to work if there is already an active session in the cart - even if in a different window/tab.

Is there a trick to this please?

Thanks!!