If your car is under warranty then the best thing you can do is take it in to the main dealer. If they agree to change the belt and the tensioner (which is probably where the fault lies) then it's a nice way of saving yourself £500 plus!
Engine belts often slip a bit and squeal when it's colder, especially in older cars. My sister's MX5 has done it for years, it's a good judge of how cold it is outside by the length of time it takes to stop squealing!