For what it's worth, about a week ago I went to my local tax office in Bangkok and told them that I wanted to apply for a tax identification number (TIN).
They asked if I had any income arising in Thailand. I replied that I did not and lived on my pension from the UK.
They said that I therefore did not need a TIN and would not issue one.
I said that I understood that from 1st Jan foreigners would be liable for tax on money brought in from abroad. They said that they had no information on this and wished me a good day.