Duncan - remember as you entered the prison level => a guard attacked you? This one drops a key for Tax. Did you free him? And the table key (west corridor) opens the cell of the other imprisoned imp. Which key are you missing?
http://beyond.divinedivinity.net/help/act2_prison.htmOk, the attacking guard at 1 drops a key for cell at 3 => Tax
The key on the table at 2, opens the cell at 6 => imprisoned imp
the key on the shelf at 4 opens the cell at 5 => monster with BF key 1