Often there are special diagrams implemented, which cover for the booked diagrams, due to revised services. For example, last weekend, FCC services between Bedford and Brighton were diverted via away from London Bridge, with an extra 15' in the schedule, so Bedford drivers were being relieved at Three Bridges, to allow them to have their PN breaks, with Brighton drivers working to / from Brighton. Extra / revised diagrams being created.
Also, if the line is completely blocked at either Brighton or Bedford, then there is extra time / duties required to allow them to go by taxi to where the trains are starting.