Update from Suvarnabhumi today: I went to the airport help desk who rang immigration. I explained the problem over the phone in Thai to the immigration officer, who told me to go up to departures and wait by M24. She came out about ten minutes later, checked the passport and took it away to get the right stamp. About twenty minutes later, she came out again, with the date corrected to 4th September, a dated and signed correction, indicating the correct visa page and the single re-entry permit now cancelled. I asked about the multiple re-entry, which was not stamped "used" but she said that this was automatically cancelled when I got the non-B. Everything should now be OK and the non-B is now valid and correctly stamped. I have to say, great service by Thai immigration - credit where it's due. Thanks
@Tod ********
for the help - I owe you a beer or two.