Hi all have just returned from a month in Thailand and have some info on the following trip if people need it.
Head to Nong Khai bus station early to catch red+white bus No 221 to Udon Thani bus station No 2. Bus leaves at 6.50am and costs 38thb.Arrives Udon Thani at 7.50am. Be sure you catch this bus otherwise buses head to old bus station No 1.All buses to Chang Mai etc leave from No 2.
Bus to Phitsanulok leaves 8.15 am and costs 240thb. There is a 3 hour trip to Loei were you can quickly go to toilet and that's about it.Quite often seats are oversold so if you are alone leave something not valuable on your seat otherwise you will be standing for next 4 hrs.
Very scenic winding trip after Loei through national parks, mountain ranges and loads of crops being grown. Arrive Phitsanulok central bus station at 3 pm. There you can grab a bite to eat.
Bus to Sukothai leaves every 1/2 hr and costs 45thb.This will drop you at main bus station were you can grab a tuk-tuk to new Sukothai costs 40-50thb.
All up takes about 12 hours. Points to remember are take all food and drinks with you,try and grab a toilet break at Loei sit back and enjoy the scenery.Long day but everything matches up and goes smoothly.