Apologies to those who wanted more land border information following the subject line.
The solution to my problem turned out to be a covid-19 extension (that had been denied for the first visa exempt extension}.
UPDATE:
Extension, 60 days, probably worked (under BKK consideration now)
HH Immigration are finally using the affidavit pages.
HH say the Covid 19 excuse will finish 24 August. I noticed there's also a box to tick if you're having trouble with flights.
AFAIK, although foreigners have visited Myanmar there, it has never been good for in-out passport stamps.
Thanks for everyone's input
Simon.