I came here to ask the same question Their use seems to be widespread but its impossible to say now whether these are simply picking up on their use here They do not appear on the sites of the BBC the Radio Time or Neal Street Productions and they are not being used on the st series DVD or the BBC iPlayer The fact that they only appear to have been added here and on the main CtM page after the episodes screened also rings alarm bells I am therefore going to delete them until someone can come up with an official source that endorses them