Sadly not a spare, just one that I can re-purpose temporarily.
It's sadly looking like some key files are corrupted somewhere. It's crashed twice again with card in different pi. Other pi scanning the folders with Plex is still working away no problem, and to further test I have a third pi scanning the folders with Subsonic over wifi - again with no problems occuring so far.
What I am doing now is installing everything that was on the faulty one onto another sd card. However, for some reason I can't get plexmediaserver to install on it:
E: Unable to locate package plexmediaserver
After comparing sources.list and the items in sources.list.d folder it seems I installed added a bunch of repositories that required public keys on my other pi's. After a lot of faffing trying to find what I needed to wget and apt-key finally got plexmediaserver installed.
No idea why as it installed fine on the other pi only a few hours ago.