Hmm ok, couple things, someone said that a fan belt drives the alternator. It doesn't, it's drives a (yep, you guessed it) fan, and hasn't been on production cars for ears.
The belt he's talking about is the alternator (or auxiliary) drive belt, now as we don't know what your car is it's impossible to say if he's ripping you off.
Generally speaking, on most cars the alternator belt is a small V shaped belt about a foot long, and runs from the crank to the alternator. It's normally a 15 minute job to pop the hood, slacken the belt, remove it and pop a new one on, and a belts about £20.
However, I've also seen a number of cars where the belts sit behind a plethora of covers, or where it's been included on a serpentine belt.
If he's charging you 125 quid JUST for the belt replacement I think you're being done, unless you have a car where it's obviously very awkward and needs parts removing.
Take a look at this imprezza, this is not a 15 minute job to replace the belt, i might swallow £125 for this but not for most cars.
http://www.flickr.com/photos/mapper-montag/398829125/