Khun Marmite,
As an interested bystander and a potential new client of "Khun" the contractor. SEA I happen to have his business card handy and my Thai wife to assist in reading the name and such on the card.
As written in english on the card:
name of the company; LUEDNITI Construction Co.
Khun's name in english spelling; Pathompong Luednitirunglid
My wife heard Sea calling him Khun also. I can see why ST has called him Khun all this time as his name is not easy to pronounce at best, so, when I inquired to my wife she says it is perfectly acceptable to speak to him as Loong which means "uncle" in Thai . My wife says this works saying "Khun Loong." We have not asked for his "nickname."