I thank you everyone - I’m going to do the Visa Exempt entry method - however I’m worrying if I need to show an onward journey. I’ve been told to just tell IO that I’m planning a bus trip to Laos, and will indeed do that, but have some anxiety!
That's really useful thank you. My next question was how long it might take to get the re-entry permit and can I get one at Bangkok airport before heading on to Sri Lanka, but I think i've found out ... Yes & allow up to 4 hours
But sounds like it could be worth trying my luck to start with an exemption on first entry and if they insist on activating the SETV, I'll go & get the re-entry.