Did this trip last year in F. It's difficult to stay awake on the LHR-SIN leg as it departs late evening when you're tired anyway. I slept about eight hours on that leg and five on the SIN-SYD leg. A shower and a change of clothes at SIN is a good idea. Try and stay hydrated,it really is a long time in the air. As much as it's tempting to have a good drink, waking up with a raging hangover on the plane is no fun, so go easy on the drinks. Even though I slept loads,I still felt grim when I landed,and the hotel wouldn't let me check in early as they were full! They did let me use the spa to get a shower but I cursed myself for not booking the room for the previous night and teeing them up about my arrival time. Have a great trip.