There is no sign of it in 'Purchases' in the app store (which is expected really) but still interested if anyone has managed to get around this issue.
Is Aperture listed in the "Accept" section, when you sign into the AppStore on your MBA? A preinstalled application should appear in the "Accept" portion of the screen, when you go to the "Purchased" tab. After you accept Aperture to your Apple ID on your MPA, you should be able to download it from the "Purchased" tab on your new iMac.
when did you buy Aperture - preinstalled on your MBA? Was it before the AppStore came into existence?
Then you should have received a serial number together with your preinstalled Aperture version. Does Aperture show a serial number in the "About" panel?
Aperture's main menu bar > Aperture > About Aperture?
Then take note of the serial number and download the Aperture Trial version from
Now run the Aperture trial installer on your iMac and upgrade immediately (using Software Update from the menu). Then launch Aperture and unlock it with the serial number.
Added: If you bought Aperture before the AppStore was available, but you cannot retrieve a serial number, contact Apple:
Pro Application Replacement Serial Numbers: http://support.apple.com/kb/HT1861
Thank you so much for your help. After some fiddling about I got that to work. I found my license number and managed to download it and get it running. I was even then able to go to the AppStore, click on updates and it found the latest 3.4.4 software, which I am currently downloading.
Thanks again for your help!