I just returned to Heathrow with Qatar and it was good, nothing special, had just enough time on a 1hr40 layover in Doha to change, no issues and cheap flights
As Todd says above, do you 1st t days with no hassle at all, come back on your 2nd visit and before your 30 days is up go to local immigration office and get another 30 day extension for 1900baht, bring passport photos, photocopies of your passport main page and entry stamp page, the resg of the forms tou can fill out at the office