UKPassats

hiya guys, hope u all had a good xmas and new year!
looking for some info and advice for my 5.5 pd130. recently in the cold weather its been making some funny sounds when starting. only when its really cold, when i start it, it makes a hissing/tapping sound for about 10seconds, then its ok! but as i said only when its cold, today it did the same but shortly after a message came up on the dash saying alternator workshop.

does this mean the alternator is on its way out, obviously the dash message would suggest this but i wouldnt of thought it would make that sound when starting. a guy at work suggest i have my belts checked to make sure they are tight and not slipping, but i only had them changed this time last year ish...

do you know how much i am expected to pay to get a new alternator fitted?! want to make sure i dont get ripped off. or is it something i can change my self, i know on other simple cars (rover) its fairly easy but vw likes to hide everything

any replies/advice would be helpful, cheers in advance, kev!