What is the most comfortable such service and why is the price of the ticket? I picked up a rumour that there now are long-distance busses from Bangkok which offer seats folding fully flat - true? to where? Chiangmai?

dont know about lying down flat but be careful; i went VIP, nice bus, comfy seat, loads of room felt i was set up for a relaxing journey until they switched on the music, it was LOUD, i tried putting in my headphones on full and i couldn't hear my oun music. i looked around and non of the thais seemed to be enjoying the racket, no one could sleep it was impossible. who knows why they do it. what a nightmare. i'd pay peanuts for a shit seat as long as i could have peace and quiet.

you get to lie down flat by train

This is the company your asking about. They have their on terminal at Arcade Bus station.

http://www.nca.co.th/index.php

and this is the bus. http://www.nakhonchaiair.com/firstclass.php

dont know about lying down flat but be careful; i went VIP, nice bus, comfy seat, loads of room felt i was set up for a relaxing journey until they switched on the music, it was LOUD, i tried putting in my headphones on full and i couldn't hear my oun music. i looked around and non of the thais seemed to be enjoying the racket, no one could sleep it was impossible. who knows why they do it. what a nightmare. i'd pay peanuts for a shit seat as long as i could have peace and quiet.

you get to lie down flat by train

I too would avoid like the plague those conditions of travel.

Only reason

This is the company your asking about. They have their on terminal at Arcade Bus station.

http://www.nca.co.th/index.php

and this is the bus. http://www.nakhonchaiair.com/firstclass.php

And worth mentioning that NCA always turn off the video and music on overnight buses at about 9 or 10 PM

And they don't stp for food at 3am, waking you up and destroying any chance of more than 2 hrs kip.

just fly, it's cheap. forget the bus. too many accidents. not worth the gamble with your life.