Hi Rob and welcome to Flyertalk.
The route you have chosen is one of the most difficult to get redemptions on and I note that you require 3 seats in Business to fly to Syd or Mel. As others have said BA only flies to Sydney so if you wish to use the 2-4-1 then you would have to fly on BA15 to Syd and BA16 back.
Unfortunately, seats are snapped up pretty quickly and I think you might have left it too late already if you want to fly out before 18 August next year as the seats are released at T-355 and go almost immediately.