Tightening Fan Belt

So I've done a search and all the ever comes up is 'Cam Belts' so started a thread...

My fan belt has a slight squeak when idling and it's doing my swede in!
I popped the bonnet yesterday and spotted the alternator, but could see no way of tightening it, as its all so squashed in there!

So anyone done it before? And please tell me it's an easy fix...
 
auto tensioner...

if its faulty its either replace belt or fit a belt/tensioner kit
 
Just a bit fiddly. You can access it through the wheel arch.easy enough to get off, pain to get back on.