Hi Keighley,
Although they have said 2-4 weeks, by the looks of past experience of people in the immigration blogs, you might still be lucky and get your visa in time to take the flights you have already booked. Since you are in the UK (and presumably British citizens), you may also have the option of getting an eVisitor visa to travel on the date you were suppose to come so you don't lose money paid for your flights. Might be worth asking your migration agent. When we were coming here for the first time, there was delay in getting the 457 visa and I considered this option.
Hope you get to keep your plans!