@Simon *******
I don't know which immigration office you use but I guess a Non-O visa being issued is discretionary and perhaps your particular office has decided not to offer it. Here's how it is handled in Phuket:
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If your local immigration office doesn't offer this service, then I guess the alternative would be to acquire the Non-O visa at a Thai consulate outside of Thailand. I believe they can be obtained in neighboring countries, but I don't have any details about that.