Thanks guys; so the general thoughts are that a 14 hour flight to SYD with a connect to MEL is better than via AKL or direct to MEL so I can avoid TBIT!
I'm arriving at TBIT from LHR around 3PM on a BA 'day flight' so unless I overnight at LAX QF8 is not an option; even then it's in March and does not get to SYD until 22:20 so really that's a NIX.
I'll book a late evening flight via SYD to MEL and try to get at least 50% sleep on a 14 hour 'night' flight.