Hi James,
For off-peak capping all journeys after 0930 count. If you did split at say Slough in one direction then that would be counted as two journeys, Marlow to Slough and Slough to the appropriate zone boundary. This could then put you nearer the cap for Marlow which will apply as the absolute maximum.
I’ll be interested to see what they say about the peak journey. It won’t matter whether the train called at Hayes or not, within the zones the calculations are all based on zones used.