Mike, you'll likely be short of sympathy here - not having either original discs or registering is more suggestive of illegal copy rather than carelessness.....Don't mean to sound rude, but those discs would have cost you upwards of £600 in 2009 and now you don't have them?
However, being a positive chap - I believe you can install FCS2 on Lion but you need a copy of the Rosetta application from earlier versions of the OS. Get a copy of Snow Leopard on disc (ebay or maybe an Apple specialist) - it'll be on that and you should be able to install just that application.
FCS 3 will install on Lion without trouble. If you get a copy just delete FCS2 and install 3.
Should you need to - Motion 3 projects will open in Motion 5 but you'll have to save them as copies or those files won't open again in M3.
I wouldn't recommend switching to FCPX part-way through a project. FCPX is very capable but very different.
Hope that helps
Thank you for your help Nigel. I can assure you that it was not an illegal copy. The discs (along with an extensive CD collection) were lost by the shipping firm I used when I moved abroad. "A likely story" I can hear people say but it is the truth.
My hard drive from my old laptop survived the spillage, could I get Rosetta from that? Or could I even restore my entire system to Snow Leopard?
Ouch! Sorry for your loss. (Final cut can be replaced but a CD collection is a bit more personal)
As far as I know you can't install an older operating system than the one it shipped with on a mac - so your new macbook has to run Lion.
Can't answer the other question - Rosetta was an optional install if I remember correctly so it'll only be there if you loaded it. Snow leopard discs on ebay for about £30.....