After a some trial and effort I now have Windows 10 installed on the MacBook Air. I'll provide the approach in a MyAnswers solutions shortly (www.OnlineConnections.com.au/myanswers) which will be available to clients.
I'd suggest at this stage to stick with Windows 7 until VirtualBox produces a stable version 5 as I've noticed a number of issues where the screen freezes or isn't handled properly.
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Update: 15 Oct 2015
I'd suggest giving the update a miss at this stage. Virtualbox freezes very frequently (even as often as every few minutes sometimes) resulting in the need to quit and restart Windows. I consider this quite poor on Oracle's part. I'll update this post once I believe Virtualbox running Windows 10 on a Mac is again reasonably stable.
Update:11 February 2016
It's been a number of months now and running Windows 10 using VirtualBox on the MacBook Air is still not stable. Fortunately my Windows use on the Mac can be limited, and for the short periods quitting VirtualBox and restarting Windows 10 provides sufficient time between freezes for me to get what needs to be done.
Update: 11 March 2016
Applied the latest update 5.0.16r105871. Windows 10 automatic updates also applied. Windows 10 still freezes. Perhaps since I'm using a Mac to run VirtualBox this doesn't have the same number of users and thus not as high a priority. It really does seem strange VirtualBox is performing so badly.
Update: 1 May 2016
Applied the latest update 5.0.20 and still freezes. I decided time to investigate as I didn't feel a problem like this would last this long. Through trial and error I found the issue appears to be related to the settings. Once the settings were changed Windows 10 on VirtualBox so far hasn't frozen.