@Graham ********
I did exactly this UK-BKK-DPS-BKK with no problem.
On arrival at BKK I explained to the IO that I had a Single entry Non O visa but that I had a flight booked to Bali a few days later, and asked if he could please just stamp me in on a waiver to save me the time and money of a re-entry permit.
He did, and suggested that when I return to BKK, that I remind the IO to activate the Non Imm visa.
All went smoothly.