Sorry foosty, i cant explain what has happened to your plec, hopefully someone else will have a better idea, but i would suggest getting a water test kit if you dont already have one, they really are invaluable when trying to work out health issues.
as an immediate treatment, i'd look at doing daily partial (15-20%) water changes for the next week, clean water will heal most injuries fairly quickly.
the real danger for him is that of a secondary infection, so i'd get some melafix and pimafix from your local fish shop (LFS) and treat with these for now, they are both all natural so won't do any harm at all to any other fish or the filter.