This is mainly valid if flying in J/F. If flying Y (as the OP is intending), then the cost savings are far slimmer when you factor in connecting flights, hotels etc, let alone the hassle factor.

Are you really set on a Singapore stopover? If not, I've seen £645 with Cathay to Syd, out 17/9, back 16/1 with the fare sale they have on. Note, later dates in Jan are opening up daily as the they on release a set number of days ahead.

I would be more focused on a comfortable ride out there and back. Cathay in Y is reasonably good with 2 hot meals on all sectors. Ideally if you can, select your flights to arrive as late as possible into Oz and back to the UK. I find it helps with the jet lag. I despise the early morning flights, especially if you land somewhere at 5am and need to check into a hotel but can't until 2 or 3pm!! Argh!