pointed out, in Sisaket same. Plus the last day of your nearly 15 month get an extension from 60 days , for visiting family ( 1900 Baht) . Have done it the last 3 years.
If it's me , I would get a flight from UK to Lao or Vietnam meet my wife over there and get my multiple entry visa there. Thai Consulat in Savanakhet or Ho Chi Minh city
do you know ,can you get this paper without the Thai marriagepaper when you have only your marriage paper from your home country ( Germany ). But translated in Thai and testified by the German Embassy 25 years ago.?