Safari Ajax "Add to Cart" (Cookie?) Issue

There appears to be a slight issue with Safari in regards to adding products to the basket with the ajax instant-add-to-basket feature.

On first visit I can add products to the basket fine UNTIL I either checkout OR I "empty cart".
From this point onwards, only one item can be in the basket at any one time. For instance, let's say I add product A to the basket. That's fine and appears there as it should. Let's say I now also want to add product B: I press "Add to cart" and it is added to the basket BUT it knocks out product A. There can only be one product in the basket at any one time :S

I've tested with no problems in Firefox and Opera. It appears to be just in Safari. I am using Safari 5.1.1

Like I say, this only happens AFTER the basket has been filled and emptied. As a guess I'd say it was a cookie issue.