I had the same problem and after changing most of the pulleys found it was the idle pulley that was squealing. This part has been modified, so may look slighly different.Cost me £17 plus the Vat from ford. The bearing was shot and there was wear on the wheel from the belt on one side. Word of warning taking it off is fine getting it back on isn't. I found I had to remove the engine mountain and drop the engine slightly to gain access to replace part. once I did this it didn't take long to put back together..and low behold no squeal. Good luck!!!
Tony
August 2008