Skoda Octavia 1.9TDi 1999 central locking problems?

When operating the central locking at the driver's door with the key, not all of the other doors operate - some may work sometimes and some all the time, but the driver's side rear door has now locked permanaently and refuses to open - even if the door lock button is pulled up manually. As it is locked, I cannot get at the workings to see what is wrong. Can anyone help?