Thanks, I copied everything to another location during the rename process.
Prelude copied and renamed the .movs from the 5D exactly as you suggested, but copied the AVCHD and kept the old indistinct names, despite me selecting the appropriate renaming options. You’re right, transcoding would have done it, but I didn’t want to make ProRes at that stage or potentially lose quality by transcoding my master files to another format.
Running the AVCHD files through clipwrap (making them .movs) and then renaming with Prelude CC and copying to another folder was the only way I could get it to work.
A peculiarity of AVCHD I’m guessing. Something to do with the folder structure. Before Prelude CC came out I tired renaming in bridge but that lost all association to the metadata within the AVCHD folder structure. I thought that Prelude would have a clever way of renaming and copying the files whilst keeping the timecode/metadata, but I think i’ve lost it all anyway. It’s not a big deal to me, but it’s certainly frustrating to loose those options straight away.