Well, for what it's worth, I started the Citroen van job friday afternoon.
Took off the alternator (sod of a job, can't get at the bolts) took it to my alternator reconditioning mate of mine, no fault with pulley, tried to fit it back on but couldn't get the bolts back in so removed the tensioner and guide pulley to test them....both pulleys noisy and tensioner didn't feel right so ordered both items.
Went back today to fit the new parts and refit alternator (got the bolts in after massive struggles) after fitting the belt I started the van but the noise was still there....I sat for a while watching the pulleys go round and noticed the noise moving from pulley to pulley so removed the belt again and went to the factors to check the part number....guess what? The belt was the wrong one that the dealer had fitted, too big, I had compounded the problem by fitting a new one based on the part number on the belt the dealer fitted.
After fitting the correct belt the noise vanished!
Sorry to waffle about this, but I just wanted to give an insight into the fact that fitting a new belt doesn't always mean the solution to a noise and that trying to diagnose a problem sometimes isn't as straightforward as it may seem.
Forgot to mention, the cost of a tensioner, new guide pulley, and new belt came to £55 (trade price, best quality Gates parts)