As Stuart says residency certificate. Get it from immigration. Don't get it at the UK Embassy they charge about £50! Immigration in Korat issue residency certificates for free with a 7 day turn around, fee and turnaround may vary from office to office. The paperwork at Korat was a bit of a faff, but if you want a bank account that's how it is...
I've never quite understood the advantage of leaving/re-entering the country to get re-stamped on an O-A 50+? A new visa costs Bt1900, it would cost me more to leave/come back. What are the advantages?