Iain: Underneath the option "Tartan Flashes:" will appear the message, "Sorry this tartan is not available in this weight.", if the tartan is unavailable in that weight. If it is available, a "Quantity:" selection field appears instead.

For example, I just selected MacFarlane Hunting Ancient and then selected in turn each of the 4 possible weights. The "not available" message appeared when I chose the 10 oz. and 18 oz. tartan weight. The "Quantity:" message appeared in the others. The picture of the tartan was present in all four selections. Hope this helps.