I managed to get my 'Podcasts' back using MP3Tag for Mac. A kind person has compiled the (windows only) MP3Tag program with a Wine wrapper that allows it to run on the Mac. It isn't fast, but it doesn't cost anything either.
I firstly deleted the affected Podcasts from iTunes 11, making sure NOT to move the files to the trash.
Open up MP3Tag (Mac) and click File: Change directory. Select your iTunes/Media/Podcasts directory.
Select each file in the window in turn (or if they are similar select a group of files).
Right click the selection and click 'Extended Tags'.
If it exists, click the 'PODCAST' entry and then click the red X to delete it.
Any other PODCAST... entries can be deleted too.
Click OK to save this.
Select each file in the window in turn (or if they are similar select a group of files).
Right click the selection and click 'Extended Tags'.
Click on the Add button (it has a yellow star on it) and then select in the dropdown box PODCAST, and in the text entry box enter the number 1.
Click on the Add button (it has a yellow star on it) and then select in the dropdown box PODCASTID, and in the text entry box enter some text. I used the artist name.
Click OK to save this.
Now locate the files in a Finder window, go back to iTunes and drag the files and folders from finder back into iTunes.
This worked for me, sadly ID3 Editor refused to load, saying my 'trial' had expired...