[ Why not indeed. What's it even matter here, it's not like they have someplace to be in the morning. He only brings one bottle though, thefted from the Blood Shack (seriously, it's more like an extension of his own liquor cabinet by now), mindful to not give Ianto alcohol poisoning on top of everything else.
The knock on the door is short and to the point. It has nothing to do with being polite - he's actually not sure if he can go in on his own still, or if he needs an invitation. ]