The actuator mechanism has a plastic clip that holds the window. When the window isn't used for a while it gets stuck in place and when you press to lower it the plastic clip breaks by the power of the electric motor. The window drops but won't come back up. The plastic clip probably costs 20p but you have to buy/ have fitted new mech which costs £210. I've had 2 go in 6 months! I think this is a well known problem and Renault won't like publicity so suggest you have a severe go at dealer to have fitted for free.
stiffybrian, November 2008