I got to my destination although I did encounter some problems.
Firstly, the staff at Fame Tours in Chumphon Town were really helpful, friendly and also could speak English well. They have a café area where you can use free wifi and get something to eat while you wait. The bus company, Rungkit Tour got me to my destination and on time. The seats of the Rungkit bus were clean and spacious and bus air-conditioned.
One of the issues I had was the toilet on the Rungkit Tour bus. I was very cramped, both in height and floor space. At 185cm tall and with big feet I could barely get in the booth. Secondly, the seat on the toilet looked like it had been ripped off so there was anything to really sit on. Thirdly the light in the toilet didn't seem to be working, so when you closed the door you couldn't see anything. These things, along with the constant movement of the bus made the toilets practically impossible to use! Mind you this was a 6+ hour journey. Rungkit definitely need to improve the cleanliness, accessibility and maintenance of their toilets.
Another issue I had was knowing where to get off the bus. This was my first time travelling long distance by bus in Thailand. The bus seem to stop at random places to pick people up, not just at official bus stations. It also seemed to drop people off at random places on request. I figures that perhaps they would not stop at my bus stop (in Khao Lak) unless I went to the front and made a request. As I am unfamiliar with the area I was needing to constantly check Google Maps to see where I was in relation to my destination. Eventually I got dropped off but it didn't seem like it was an official bus stop or station, just a strip of lawn about 50m away from some shops. Eventually I found my way to the hotel via hailing a taxi from the road. I should also note the bus staff didn't appear to be able to speak any English, which in itself shouldn't necessarily be expected although it certainly would have been useful in working out where and how to get off the bus.
Lastly, the ticket for Rungkit said "Free Wifi" but I checked for wifi on my phone and there were no available connections. Not a huge issue but just added to the confusion of the trip.