hello james
found a couple of options for you.
1. go to british airways site www.ba.com they have a flight from lhr - mel on the 28th feb and rtn syd - lhr on the 12/3, the price incl. of all taxes etc... is £940 and then go to https://book.jetstar.com and book an internal single flight from mel (tullamarine melbourne) - sydney on the 9/3 for AUS$49 (£26)
2. go to http://book.qantas.com.au and you can get a multi leg flight on the same dates as above lhr - mel then (avv melbourne) - syd and then syd - lhr all for £929 incl. of all taxes. with going to qantas's site the jetstar flight is included in the quote. with qantas with flights to and from ozz have 1 stop and with flight times of 22hrs 30mins and 23hrs 20mins. instead of booking separatly with the ba price above and visiting 2 sites, qantas is a 1 stop shop for your flights.
3. http://www.singaporeair.com singapore airlines have a similar option to the british airlines site all on the same dates rtn inc. of taxes etc.. is £870 rtn + the £26 from jetstar and similar flight length times to qantas with 1 stop in singapore.
but the best price is.
4. with http://fly.emirates.com all the same dates again but on the outward flight to melbourne it is lhr - dubai then dubai - singapore and then sing - melbourne. the flight time is the same but there is a 12 hour wait for the connecting flight to singapore, 2 stops instead of 1. on the rtn flight from syd - lhr there is the 1 stop in singapore with flight times and only a 90 minute stopover before getting the connecting flight to lhr (similar flight time with qantas). all for £807 incl. + the £26 for the booking with jetstar
the most expensive option is with BA, total price £966 and the cheapest is with emirates at £833, (£764) with etihad below.
hope i have given you some food for thought. if it was me i would book sooner rather than later as prices in my experience tend to increase nearer the departure date.
cheers
jim
just found a flight with etihad airways via opodo http://www.opodo.co.uk (multi-stop flight search) same dates again lhr - mel via abu-dhabi (1 stop)and the same stop on rtn from sydney - lhr. the flight time are 22hrs 55mins and rtn 25hrs 55mins (very good) and the price incl. of all taxes etc.. is £748 + £26 (jetstar) = £774
go to http://www.etihadairways.com and get the flight for £738 + £26 = £764