After installing R4V3 with the auto-upgrade (previously had R4V2) I've lost all my recordings.

They still appear on the list in myth and the NUV files are on the disk but myth refuses to play them.

If you try and delete them, they all show as zero bytes in length!

Any ideas?

Check out this thread and see if the problem sounds familiar.
http://mysettopbox.tv/phpBB2/viewtopic.php?t=1293

The basic problem is that the KnoppMyth CD doesn't let you fix the time so that the restore happens in the wrong time zone. To fix it:

1) Make sure that your timezone and time settings are correct

2) As root, re-run mythrestore manually.

This worked for me...