I have had my 2003 TT from new and have always used the same key to lock and unlock it. This key is now becoming increasingly unreliable. I have dug out the spare which is working perfectly.
Just in case the spare does not provide seven years service, or in case I lose it, I would love to know how to get the remote key open to change the battery. There is a small rectangular notch on the opposite side to the buttons, the side with the Audi logo, but it seems as if it is going to take some force to shift it.