I flew this exact flight, sitting on the left side in Y.

We departed on-time. The flight encountered problems about 30 minutes after take-off. I first noticed it turning east and then the fuel dump began. The captain made an announcement shortly after that there was a technical problem and it was under control. The fuel dump would last 45 minutes and we will return to Bangkok.

There was no panic onboard. It was quiet and the crew did not buckle down. They would serve drinks as the fuel dump continued.

The flight path image on the news website was accurate. We dumped fuel NW of Bangkok and we did not reach Burma.

Upon landing in Bangkok at 3pm local time, we parked near the maintenance area. I heard the crew mention they are organizing a spare aircraft and we would continue our flight on it. We were sent to the terminal and there was a sign at the gate that we would re-board at 4pm.
-
We got on the buses again long after 4pm and re-boarded another A346 and took off again after 5pm. We landed in London just after 10pm GMT.

Seems TG did a wonderful job getting the spare craft ready within an hour of us touching down. The final arrival time wasn't too bad at about 2.5 hours delayed.