I tried all the usual things - new belt (twice - both sizes :-), new bearing, cleaning, dressing, grease, wd40 etc etc etc.
All worked to a certain extent - but the problem always came back within a few hundred miles.
However, I then tried the "penny" trick, and it's been OK for 4000 miles.
The problem appears to be that the axis of the idler wheel is not exactly parallel to the axis of the other pulleys. This is probably due to wear in the idler mechanism.
To test, get a ring spanner that closely fits the idler nut, and with the engine running place it vertically over the nut. Grip with molegrips and gently twist anticlockwise (looking downward)
WARNING. Be careful. Fingers are not a field-replaceable component
If the squeal goes away when you twist the axle, try the permanent fix. Loosen the tensioner mounting nut (not the idler nut) by a couple of turns, and fit a spacer behind it at about "9 oclock". A large washer or a coin is ideal. You may have to experiment with position and thickness.
Bruce.
