I'm travelling from Oxford to Brighton via the North Downs Line next week using a combination of two off peak day returns (splitting at Dorking), and journey planners all recommend changing at Gatwick for the fastest journey. However, given how Gatwick Express services tend to be more expensive, I'm concerned that my ticket from Dorking to Brighton might not be valid (especially since from Gatwick to Brighton, Gatwick Express tickets do command a premium). Would I be able to make this journey without risk of argument, or should I stick to slower trains?