Phone Trailfinders in Kensington High Street, London. They tend to have the best deals. You could also try Austravel in London also. Things are not as cheap as they used to be a couple of years ago as many airlines are reducing long haul flights to concentrate on transatlantic routes. If you have time for a stopover you can get some great deals - Singapore airlines will give you a three * hotel for £1 per night if you book a return flight with them to Australia. Malaysia airlines also tend to have good stopover deals.
Don't forget to enquire about the availability of airmiles - its really worth it for long haul flights. One return flight to Australia with Singapore airlines will give you enough airmiles for a free flight to Paris. Two flights to Australia will give you enough for a free trip to Orlando!
Cheaper airlines tend to be Korean Air, Garuda Indonesia and Japan airlines but their inflight services tend to be pretty awful at times. Japan airlines will try to temp you with the offer of a 'free stopover in Tokyo' but this is a bit of a con. In reality you arrive at about 11am, are bussed to your stopover hotel, and collected at about 7am the next morning for delivery back to the airport. If you want to stay for another night to actually see some of Tokyo they will charge you full price for that 'free' first night.
You should also try Canadian airlines, Air New Zealand, Qantas and Cathy Pacific and British Airways. Don't forget that cheaper airlines will usually have more stopovers and the flight will take longer. So with the more expensive ones you will have two flights of about 9-12 hours each, with a 3-6 hour stopover in the middle. A Cheaper flight can have as many as 4 to 6 stopovers and you can end up travelling for up to 36 or 48 hours to make the same journey which will take 24 hours if you pay a bit more. Don't forget to really push the
travel agent to find a special deal because they tend to look at only a couple of airlines unless you make them look up the deals for all of them. They also tend to overlook the stopover deals so ask them about these also. And they are pretty ignorant about airmiles so hassle them about these also. Some airlines will not offer airmiles with the cheaper deals but more and more are lately, so it pays to ask.