My 2 cents worth:
1. I have done comparisons many times including noting down specials in Sydney before flying out of Sydney. On every occasion I have found that Sydney duty free alcohol is substantially more expensive than duty free alcohol in Singapore so if you're flying through Singapore back to Sydney buy your alcohol there. Note that you need to buy in Singapore at least 1.5 hours before your onward flight.
2. If you want to take wine overseas buy it at a discount liquor store before you fly and pack it among your clothes in your checked baggage (in a sealed plastic bag to be safer). It will be at least 20% cheaper than buying at Sydney airport duty free.
In your particular case it would be best to buy the cognac in either Singapore airport or London airport.
1. I have done comparisons many times including noting down specials in Sydney before flying out of Sydney. On every occasion I have found that Sydney duty free alcohol is substantially more expensive than duty free alcohol in Singapore so if you're flying through Singapore back to Sydney buy your alcohol there. Note that you need to buy in Singapore at least 1.5 hours before your onward flight.
2. If you want to take wine overseas buy it at a discount liquor store before you fly and pack it among your clothes in your checked baggage (in a sealed plastic bag to be safer). It will be at least 20% cheaper than buying at Sydney airport duty free.
In your particular case it would be best to buy the cognac in either Singapore airport or London airport.