flight help

[mic_check12]

I'm currently backpacking round asia and plan on being in Melbourne, Australia in early december. I want a one way flight back to England (preferably manchester but I'd take london if its cheaper). Is my best bet to book online now/online nearer the time or with a travel agent in melbourne when I get there? (this will only be a week or so before I need the flight). I'm interested in it being as cheap as possible. I've found one for 600 pounds but I'm sure it could be cheaper. Any other ideas? I want to fly on the 7th but could go a day or 2 earlier, no later. Cheers for any help.