Fiddling around with the wayback machine I found this, when did they stop passengers using advance tickets from travelling on the next service if the train was overcrowded?
NRE (archived)
Buy in Advance; Train Specific tickets – includes Apex, Bargain, Advance, Super Advance, Value and other Single & Return tickets requiring re-booking with reservations on specified trains
If at the start of, or during, a journey a passenger finds the train or trains specified on their tickets are cancelled, excessively delayed or overcrowded they may use either the next available train or, if this is not possible, any other train to travel to their destination.
NRE (archived)