Hey cw-kid, the chances are its the audio as Ian said, however FIRST you should see if Virgin are setting the copy protection bit on that specific channel, this is a per channel setting and is not the same copy protection as OCUR content as you may hear about from the US. It is possible they are using it in the UK which restricts playback of these files on machines other than the machine that recorded them.
Please read this blog post for more information : http://blogs.msdn.com/peterrosser/archive/2005/10/20/483294.aspx
This can effect Zune 2 being able to transcode the file for playback on your Zune device. Especially if your Zune 2 software is not running on the MCE machine, however even if it IS, i'm not sure if the Zune 2 looks at what your doing and says nup you can't do that. First thing to find out is if Virgin actually set the (or just started setting) the CGMS/A flags on Line21 of your analog signal. Now in Europe you also have something else called the CP flag but I don't know much about that I'm afraid.
Now assuming its not a DRM issue, and it is audio..
I don't know the format of Virgin 1 audio/video streams as I have no access to it, but you can check yourself by running Graphedit and seeing what type of filters are being used for the audio.
http://www.wmwiki.com/mcored/downloads/GraphEdit.zip
Now open a file which works ok, my guess is you'll see the audio go to MPEG1/DD and video go to MPEG2.
Now open your file which doesn't work and I think your going to see audio go to AC3 and video go to MPEG2.
Now the bad news, Zune 2 automatic conversion doesn't work for DVR-MS containers with AC3 audio.
I just did a quick search on the net and found this guide for using a 3rd party app and AC3 decoder to get around this : http://blogs.msdn.com/gduthie/archive/2007/12/11/zune-2-and-recorded-hdtv-from-windows-media-center.aspx
You may find other better solutions if you take a look around.
Good Luck :)
Steve