ok so a little update...
Am now back from Australia, which was amazing. Am SOOO glad I took the Phantom - the places we went to film were absolutely stunning, and I got some incredible footage and photos. I'll put some up here as a sample...
Flight was fine. I went for the hard case in the hold, for all the reasons above. Didn't seem to be a problem, and they put an airline 'fragile' sticker on it. Both ways they asked me what it was and when I said it was photographic equipment they took it to a special area for it to be loaded on separately. I had a trakdot inside the case, so could track it at each stop-off and airport we got to. This was a great re-assurance, especially last night at heathrow when we waited 40 minutes for our luggage.
Having said all that, my fears about the hand luggage were not tested - they didn't weight the luggage or measure it, and as we had a whole middle row of four on our plane, there was more than enough room for anything I could have carried on. In fact as we were with two children I had two large rucksacks (on my front and back) as I boarded the plane and nobody batted an eyelid. I think you get away with a lot when you have young children with you. Still... I'm happy with my decision. I was only swabbed on one of the 4 security checks and they seemed perfectly happy with it. Have to say, though, that four lipos in a fireproof back look exactly like a bomb vest on the X-ray. I don't know how those guys can tell the difference.
All in all, I'm delighted with the trip. It was good having a backpack for beach days when I might/might not use the phantom, or for a trip to Rottnest Island where there's no cars. otherwise I used the hard case as it's so quick and easy leaving the props on. Only problem with the trip is that I'm now tempted to go full blast and get a Inspire Pro X5 for better stills. the stills I have from the phantom are certainly amazing, but they are not anywhere like the stills I got from the same locations on a D750 (obviously). Some serious head scratching to do there!
thanks again for all the advice - goes again to prove what an invaluable resourse this site (and Inspire Pilots) is.