Characters being locked in a room seems to be one of the most recurring stories in Doctors! I've only started watching regularly since September last year and off the top of my head I can already think of several episodes where characters were locked in a room. In today's episode Julia got locked in and has been locked in before in the episode where the old guy's wife had died. George got locked in a room by a mother who was scared of her son being taken away, and then when she went to meet Prince Charles she was locked in the toilet. Michelle tried to help the prostitutes who were trapped in a house, Joe helped a woman who's husband wouldn't let her leave the house...

And that's all just off the top of my head, there are probably more examples.