GTR are doing their standard thing of trying to make everyone use the railway service instead of the replacement buses.
They do this by putting buses from Three Bridges to Preston Park in, and Balcombe to Brighton. In reality, these are one bus from Three Bridges to Brighton rather than two separate buses, but journey planners won't know that!
A search on the forum's site with "via Haywards Heath" will do the trick:
TrainSplit is a free service that makes it easy to find the cheapest train tickets for your journey.
tickets.railforums.co.uk
You get the choice of a bus to Brighton (not two buses!) , or a bus to Lewes and train to Brighton.
