Does Aperture still run on your original MBP?
Then you can fid the serial numberin the "About" panel.
From the main menu bar: "Aperture > About Aperture"
It will show you the Serial number below the headline "Licenced to"
You can use this number for your new iMac.
Added: If you do not have a running Aperture version on another Mac, contact Apple:
Pro Application Replacement Serial Numbers:http://support.apple.com/kb/HT1861
just about to wipe the MBP think i will wait a while
That will definitely be better. Or keep a bootable clone of the current install as a backup, until you are satisfied, that the new mac is working. With a bootable clone you can always sturt up from the external drive, if you find that something did not migrate properly or is incompatible with the newer system, like device drivers for printers, scanner, or plug-ins.