I took it to a repair shop. He just put a massive allen key in, like 12 " by 4". The small round thing in the bottom wasn't a screw at all. The repair guy held it in his left hand and hit the allen key with an axe handle in his right hand. So in other words there is no locking screw inside this chuck for this model drill. Hope this helps anyone else in the future.
PS As you look at the drill from the front ( bit/chuck end ) the chuck screws on clockwise and off anticlockwise.
Matt
Matt
May 2008