Not muddying the water just asking a question based on the OP that the replacement bus was from Haywards Heath but it isnt as it is going from Three Bridges which changes the picture completely.
the OP original post is untrue as it is bus from Three Bridges and Haywards Heath is not accessible not as stated Haywards Hearth to Brighton it seems.
Are there not trains to Eastbourne via Lewes,
ps Just checked national Rail it bus from 3 Bridges to Lewes and the EASTBOURNE train is the same bus replacement. So it is Haywards Heath that is not served by train.
Arent the trains going from Haywards Heath to Lewes if so that would be easier probably as you can get on at Victoria and stay on and the same on the return.