SIN-SYD is also the 'short' sector and I always prefer to stay up to increase the possibility of sleeping on the long haul sector. Sleeping on this sector unaided usually results in finding it hard to sleep on the next sector and therefore makes it difficult to stay awake when arriving back into LHR.

I always find it best to try and stay awake as long as possible the first day back into LHR as it makes getting over jetlag so much easier.

Having done these sectors on countless occasions (both pre FT as well as post FT ) and tried all the various combinations, that would be my advice regardless of what cabin you are travelling in.