I managed to get some time over the weekend to have a play in the webadmin, and managed some success with this. I've posted this in case anybody else has similar problems where their NAS is added ok, but their media doesn't show up in the orbiter...
I found that in Webadmin you have an option under 'Files and Media' for 'Media Files Sync'
In here you can select your NAS drive and it shows whether your files have been added to the database. Mine weren't, the little symbol next to them was of a drive, not a little check-mark (tick). By clicking the synchronize button, MCE popped up another window and did some synchronising. This actually took quite a long time as there were a lot of files there. However once it was finished, I reloaded the router, regened the orbiters and the files appeared. I had even tried copying some movies straight into the \Home\Public\data\Videos directory previously and they still hadn't appeared, but after synchronizing they appeared too.
I took the opportunity to get the coverart for my missing dvd rips too, although you'd be amazed at some of the movies that aren't there! Mission Impossible 2 for example isn't there, and even the search couldn't find it. Anyway, if you've come across this thread because you're stuck with your own NAS problem, I hope it helps.