Forget all this nonsense about spraying various chemicals onto the belts/pulleys. It will work for a few days/weeks but the squeel always comes back again. I have had this problem on both my 300Tdi's (Defender & Disco) and despite trying new tensioner pulleys, new ancillary pulleys, so called anti squeel belts, etc, the only solution I found that actually works Permanantly is to place a 1mm thick washer at the 9 o'clock position, between the tensioner housing and the coolant pump support bracket. What this does to the belt Im not sure but it has definately worked on both my vehicles and has had no undesireable effects.
Cheers
Log