They sometimes don't work correctly, fold-in and then back out or vice versa. The problem is the little microswitch housed inside the casing behind the mirror. The microswitch is made of plastic and the lever on it is made of metal. Over time the metal grinds down the plastic pivot ridges causing the lever to dislocate. The mirrors can’t send the correct signal back when they are asked to open and the car puts them back to the original position.
I have included this photo to show where to prise the plastic cover from. You can see the catches on the plastic and the surround housing. They need to be got at with a small flat-head screw driver and tons of care not to slip, dent or scratch the paint/surround:
Second picture shows cover off and location of microswitch housing, underneath this is the switch. It may have moved so be careful when pulling this housing off. To pull it off, use an upward motion:
This is the money shot. You can see that the housing has 2 locating pins, these go over the microswitch to house it correctly. There is a small metal lever arm attached to the left of this switch and it is this switch which has moved to cause the problem. It needs to be placed back into position correctly so that when the mirror motor moves that pushes this lever arm, which in turn pushed the tiny switch under it. That signal then tells the car its mirror positioning. If the switch does not get pushed, no folding mirror...
Use a hot glue gun or such like to get the lever back in place. Get it as weather proof as possible.
Courtesy of saphireblack we now have a link to the microswitch here:
http://www.access-electrical.co.uk/shop/view/-18453
what exactly we have to do once we have located the problem. 
