possible alternator failure?

[Niall]

Well after 4 years of trouble free peugeot motoring, I have finally broken down! Luckily only on my drive.
Cars been acting a bit funny over the past few weeks. Bad running, lights go dim when i use the windows and then today the dials were very dim and stereo was occasionally cutting out. Took it for a drive today, got back parked up and switched it off. Went out to it about 20 mins later and again, all the electrics were lazy and dim lights and when i started it, it barely started. Put my meter over the battery on idle and had 13.1v and a slight wine from the alternator area. Turned it off and checked that battery voltage and im getting 11.1v. Normally i would just say oh the battery is buggered/flat as the idle voltage would be low until its charged up then it should reach 14.4 but weird thing was, after a while the voltage started moving all over the place without any change of load. Was varying by 1v either way.

Bad battery (which would be a result as its still under warranty) or bad alternator?