Originally Posted by Sam Bee

I'm not sure if still an option, give them a call to find out. But you won't get it on the return as the shortest connection is same day from BKK-CGK-LHR, but you chose to save money and book a cheaper, longer overnight.

BUT - they did use FM7 (at least in Y) - it's peanuts to book yourself in there and a good option (they run a free shuttle to / from the airport). No onsite hotels connected to the airport at T3 (there was a fairly grim one at T2 that closed for a few months and was overpriced). The ex-Sheraton Bandana (now just Bandana Hotel) is the plusher option, now cheaper since they got rid of the Sheraton branding.

EDIT - appears to be open and with some cheap offers. Maybe someone can talk about how it is inside these days... Jakarta Airport Hotel Official Site Soekarno-Hatta Airport Hotel