Rahly, the person that created the (unofficial, since Ubisoft didn't bother to fix it) patch for this problem has uploaded the files to a nbew location. Here's a lift of steps you need to follow that should fix the problem (in most cases):
- download the "BGAEFix" program here: http://dl748.com/bgae/bgae.php and copy the exe to the main directory of the game.
- save in the same area where the bug occurs, exit the game and start the BGAE Fix tool.
- select the save game you just made, and click "Central Hall Fix".
- Start BG&E again, load the save game and now you should have the key in your inventory. Give one to Double H and progress through the area.
- You'll eventually return to the same area where the bug occurred. Make a new save game there.
- Exit the game, use the BGAE Fix tool, select the save game and click "Central Hall Swap".
- When you load the save game, you should find yourself in the area before the Central Hall and should be able to progress without further problems.