There is an amount of discussion above about how the guard signals to the driver that the train is ready to depart.
I was wondering today, as the train arrived in the station, stopped, then shuffled forward before the doors opened, how would the guard know when the train stops that the driver will not make a further movement, and they may open the doors?
The guard would/should know that the train was in the correct position before opening the doors, once the train had stopped, fully or partin a platform then the driver should no move without a bell from the guard. Similarly once the bell has been sounded then the guard should never open the doors without speaking to the driver first. This was written into the rule book and specifics such as signal failures or stopping short to retrieve articles from the track were mentioned.