Hey Adrian. I didn't need anything like that actually, but I just needed a normal non o. I just showed them my marriage certificate and bank statements. The 1 year multiple entry cost 125 pounds and you can collect it the next day.
From my understanding, you can do 2 entries per year calander year by road and there is no limit for entering by airports. But some more information about what your enquiring about would be useful as different visas could be extended without doing border crosses etc.
Hi Koos. I literally just got one from BKK. Took 10 minutes. Was very easy. Just download a TM.8 form I believe and fill it in before arrival at the visa re-entry permit kiosk near passport control after security :)