Nope, it's NOT a stupid question at all, but I dunno what the answer is..
Now were a betting man; I'd bet dollars to durian; if you got your original UK marriage certificate translated into Thai, got that certified as a real translation by the Ministry of Foreign Affairs, went to Savannakhet with it, a copy of your wife's thai I/D and the entry of her name in the thai house book, it MIGHT fly <- the operative word is "might".
I goggled err googled around but I couldn't find a single post about someone going to Savannakhet and pulling a year long multi-entry Non-Immigrant Type-O visa based on marriage to a thai who used a foreign marriage certificate, even one translated into thai and certified by the MFA.
Conversely, if that doesn't work, you could always go to your local Amphur and register your marriage here in thailand too. I think it cost 100 baht.
Then again, why not have a thai call the thai consulate in Savannakhet and ask;
Royal Thai Consulate-General, Savannakhet
P.O.Box 513, Kaysone Phomvihane District, Savannakhet Porvince, Lao PDR.
Tel. (856-41) 212-373, 252-080 Fax. (856-41) 212-370, 252-078
Sorry I couldn't help... Plus I don't know if that's the "new" building in Savannakhet or the old one..
Good Luck