What stamp will I receive upon re-entering Thailand from Cambodia on a UK passport, and can it be extended?

Hello. Looking for advice. I'm travelling to Thailand, arriving 13th February. I stay a few days then join a 29 day organised tour (Laos, Vietnam, Cambodia) and re-enter Thailand via Cambodia land border. I'm on a UK passport and will be having the 30 day visa exemption. Will I get a 15 day stamp when re-entering via Cambodia? Can I then extend this in Thailand? I've read conflicting reports...some say I'll get a 30 day stamp, some 15...some say I can extend for 30 days, others say 7 days. Slightly confused!