Hi all,
I recently had my alternator fail & whilst I was expecting it to go at some point, never knew what the signs of it going are. Scoured the web for a bit I found that one of the telltales is when the alternator produces a whine and your car sounds like you have a supercharger - which I definitely noticed but didn't think of investigating further.
So if your car starts sounding like it's got a supercharger, start looking for an alternator.
I got a rebuilt one from Ebay for £125 including postage and had to send the faulty one back (they held a £30 deposit until they received my faulty one).
Labour cost was £160 at a local mechanic and he said the aircon compressor had to be removed to get to the alternator (also had a rounded bolt apparently so this may have added to the complexity of the job.
P.S. Once the alternator goes (all electrics started playing up - radio turning on and off, lights on dash etc) the power steering goes with it too. If you're driving at motorway speeds all systems come back, but once stopped at traffic lights it all goes off too - mine went bad just as I was joining the M6 but managed to make it to the mechanic after wrestling with the steering wheel.