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The are several daily buses, as well as mini-vans, that go past Khao Sok NP (main area) to Surat Thani. Your guest house in Khao Sok can tell you the current schedule, when you get there. They are not really reserve-able in advance, but getting onboard is not usually a problem.
From Surat Thani you can then pick up the train or regional buses to Bang Saphan. You can find the train schedules at the SRT (State Railroad of Thailand) website: http://www.railway.co.th/checktime/checktime.asp?lenguage=Eng
Buses will be more catch as catch can when you get to Surat Thani.