If I arrive at bkk in December and get a 45 day 'visa exempt' stamp, am I then guaranteed to be able to get a 30 day extention?  If the answer is yes, is it therefore safe to turn up at LHR or Gatwick having bought a ticket that returns me 75 days later?

That would be up to the airline.

Seems you have read the threads that discuss this.

You could opt for "rent of flight" which involves companies such as "onward flight.com" etc.

 

There are many reports of not even being asked for onward flight.

Also reports of being asked for onward flight and with the (previously) 30 days visa exempt entry and airline being strict.

Also reports of airlines accepting that extension can be obtained.

 

So basically guess work.

Mention your airline and you may receive a first hand experience. 

 

 

I arrived in Thailand beginning of this month with a return ticket of 55 days . I was asked at departure by airline my onward flight and show it to them . I did book a refundable flight out so no problem , but i was certainly asked . Arrival in Thailand , officer asked me how long i stay and i answered 28 days since i will go get visa stamp , but no need to show anything there .

IOs will hardly ask for onward flights but airlines may ask. It is like hit and miss. I have travelled extensively in Europe with a US passport and asked for onward flights thrice in my life. Again as I said it is hit and miss. 

15 minutes ago, Onerak said:

Again as I said it is hit and miss

If the flight is overbooked it's a great chance for them to kick you out or force pricey rebooking on the spot (like me with a 32 day booking, long ago).

1 hour ago, LisuLover said:

  If the answer is yes,

Regarding Thai immigration.

Very unlikely that it would be denied.

Assuming you have no history with overstays and the like.

And for the extension you should be able to show the outbound booking within the extension period.

In practice, getting the 30 day extension is pretty much guaranteed, so you can stay for 75 days.

 

Unfortunately, that doesn't change the requirement that you need a flight out when entering visa exempt, within the time you're originally permitted to stay that is (45 days in this case). If your airline enforces this requirement, that's what you're supposed to show.

On 10/19/2022 at 7:36 AM, Onerak said:

IOs will hardly ask for onward flights but airlines may ask. It is like hit and miss. I have travelled extensively in Europe with a US passport and asked for onward flights thrice in my life. Again as I said it is hit and miss. 

Philippine Airlines asked me for it. Told them I didn’t have one. She made a call and let me go. Bangkok didn’t ask

I have now bought a 72 day return ticket with Thai Airways (LHR-BKK) so I will be dealing with them at check-in, or whoever they employ to check in their passengers.  Anyone with experience of Thai check-in at Heathrow?