where we board the bus and depart 7:42. It takes a while to get out of Phom Penh (slow traffic). Stop at Phnom Penh Bakery 8:23-30, stop for toilet and lunch 11:01-28, arrive at Koh Kong bus station (just a field and a roofed waiting area at 11.6195648,102.9857148 about 1km by road to the bridge) 13:31. Apart from aircon too cold, a quick and pleasant ride that was not delayed by medium rain. (Insert is information table I saw at the bus stop, don't know if this information is correct and complete.)
View from Cambodian side:
Immigration departure Cambodia 13:54-56, immigration arrival Thailand 14:04-07. The fastest and most pleasant land border crossing I have experienced in South-East Asia (comparison with Nong Khai to Vientiane twice, Vientiane to Nong Khai, Sadao in both directions, Poipet to Aranyaprathet, Luang Prabang to Chiang Rai, Phnom Penh to Saigon).
View from Thai side:
For my questions before the trip and report as above but without pictures see here: http://www.thaivisa.com/forum/topic/932256-cham-yeam-baan-hat-lek-trat-border-crossing-experiences/
Update 6sep2016: just learnt of https://www.camboticket.com
who give Rithy Mony and Virak Buntham as bus companies that run Phnom Penh - Koh Kong buses (with times and prices).