We went to Oz last year and did the following. I assume you will be leaving UK about 11pm and arriving Oz about 6am with a stopover somewhere around Bangkok. On the first leg, have the lovely airline meal (!) then try and go to sleep, as you should be tired anyway. I'm terrible at sleeping on planes and still got 5 hours sleep. I didn't manage to sleep on the 2nd leg, but if you can , all well and good. When you arrive in Oz, try to not go to bed until about 8pm local time. Hopefully, the next night you should be able to go to bed at a normal time, but if you are a bit tired, just go to bed early. It took us 2 nights to get to normal. BTW, you will feel like a zombie on arrival at Oz. Also, if you can afford it, I highly recommend upgrading just one class to try and get a more comfy seat. If you can check in online, do this and try and get a seat behind a bulkhead. You will normally have a bit more legroom, but will definately not have someone reclining their seat into your face!