Well in the end i've opted for an ex-LHR (rather uncharacteristcally for this board); not because i'm scared - indeed, the very helpful Rebecca at Amoun Travel had the trip half-booked for me already, but through some significant time spent pondering options i've spent about £800 for the following:

• LHR-HKG (WT+) with BA
• HKG-AKL (J) with CX
• AKL-SYD-HKG (J) with QF
• HKG-LHR (J) with BA (MFU)

Whilst nothing to scream about...

1. the MFU pays for itself (W roundtrip to HKG is ~21,000 BA miles for me as a BA Gold) and the W return is shareholder discounted saving about £85.
2. I'm just beginning to realise how useful being AA Plat is; miles are easy to earn (200% flown) and cheap to redeem (eg. BA F to HKG is 240k BA miles but only 140k AA miles, same availability). In this case I redeemed 70k AA miles + £14 (!) for HKG-AKL-SYD-HKG (>11,500 flown miles) in J.
3. I have a two-sector space-available upgrade voucher left over from the January carnage and cancelled trip to DFW which should hopefully see me in J/F to HKG rather than W/J.
4. I get a two day stop on the return leg to break the journey and do some shopping in HKG, and allows me to pay The Pier a visit
5. I get to try CX and QF (admittedly only in J) as desired