Thanks for that. It's not linked to any work, just an old man enjoying his twilight years in the Land of Smiles.
The re-entry permit was issued at BKK so theoretically I could stop by the extension desk and ask them to cancel it before I fly to Vietnam.
Actually, I would like the multi-entry Non-O to go in my other UK passport but I ran the gauntlet of Immigration at BKK about 7 years ago when I tried a visa waiver entry on that passport. The IO started gurning as his database showed I had the other passport, had an extension, was married (used to do Mar. Ext. so in their database), lived in Udon, etc.. His supervisor was even worse and thought that one of my passports was fake. The upshot was being told if you have a passport with a valid visa and/or extension in it, that's the only one that's allowed for entry to Thailand.
YMMV
TiT