Ok, I've had a look around and it's possible to open a link in a parent window from the child window and close the child window at the same time (
http://www.dynamicdrive.com/forums/archive/index.php/t-14774.html ) I haven't tested it but I'll give it a go.
So, I could probably use that mod in the way that I want. But does anyone know of a way to make the "Add to Cart" button link to a "Product Added" page without using a mod?