Galaxy 2002 1.9Tdi Alternator

In need of some help, I've got a 2002 galaxy 1.9tdi 115bhp manual and have been having some problems, there was a noise when on full lock and at low rev's which I took to the ford dealer and they said it was the aux belt tensioner which I had them change at a cost of £150. It seemed to cure the noise for a couple of weeks, I took it back again today and now they say the alternator is failing and that needs replacing as well for another £240, I want to do it myself to save some money, the main thing I want to know is how do you slacken the belt tensioner, I've read somewhere that I need a special tool and to turn the bolt clockwise to slacken, but where am I looking, can't see or feel anything to turn, and can you do it without a special tool, would a torx bit or socket of some kind work?

Also will this actually solve the problem and would the tensioner need replacing again?

Any help very much appreciated!