Probably worth mentioning that the main entrance to the Qantas lounge isn't always obvious when transiting (it's a bit tucked away and seems to be more aimed at people departing HK).
You can wander round gate 15/16 for a while, looking up at it, trying to figure out how to get in (there is a backdoor up one of the escalators but that's more for people leaving).
Its back up the central stairs, to the left as you are looking at everyone arriving out into the airport.
From the Wing F its on the same level - you just have to navigate across the busy food court area.
Ps. You'll loose any duty free you buy in Heathrow unless its in a proper tamper-proof bag!