Educated estimates:
At VW dealer ?200-?250 ish
Indendant ?150
Wherever you have it done, make sure genuine parts are used! If you use an independant you might as well buy the belt and tensioner yourself, just for piece of mind. If you're cherged much more than 2 hours labour, someone's having a laugh.
If money isn't a problem i'd get it done, just to put my mind at rest, if cash is tight take it to a local garage or two and ask for their opinion.
Just out of interest, what is the rest of the service history like? When was the last oil change? Without meaning to sound condescending, have you checked the oil level? Just thinking of other reasons for rough sounding.