Thanks to everyone who's responded and the good advice. On this occasion I've decided that what will make me feel most comfortable is to book the refundable fully flexible Emirates ticket. I'll choose a date somewhere near the end of the 60 days tourist visa which will mean I've actually got a ticket out of dodge should I need it at short notice, although the plan is still to refund it and purchase a cheaper one at some point.

The TDAC, Thailand Digital Arrival Card does asks you.  I had a friend come in December he left it blank he said they asked him about the out bound flight for the first time in over a decade of coming here. He got in without much trouble saying he was going to travel around and wasn’t sure of the exact dates but they told him in future he should book one. 

Unless you have history of overstaying you will be fine. I've been asked once or twice when I'm leaving Thailand and have showed them my (genuine) return flight booking in an app. They're not going to get it forensically analysed, all they want to know is if you're a genuine tourist and not going to overstay. If you're really worried then book the cheapest flight you can find to Cambodia, Malaysia or Indonesia.

Thanks for that, have had a look and it does seems a straightforward inexpensive option. My concern would be getting rumbled at Thai Immigration and then as you've basically cheated the system getting refused entry. Has anyone heard of that ever happening ? Just checked my last Digital Arrival card and onward travel is definitely required on the form.

Try this: https://onwardticket.com/en