We have found significant water in the wall cavity of an outside wall with no apparent pipe leakage. We urgently need to find the cause and remedy it, in a French "villa"-type house built about late '70's. The insulation in the cavity is fibre-glass. Any comments or suggestions?

Hard for us to guess where the leak is based on that information. Have you removed all of the drywall and insulation from the area? If so inspect for any gaps in the exterior skin from the inside. Also inspect the exterior to see if there is a place where water could be entering. Look for cracks and sealant that has failed.