Had a look at the black box and as usual things weren't what I expected. The reason it had stopped working at all was a blown fuse. It still doesn't work as 'one touch down' except while I'm trying to measure the voltage going to the window lift motor, i.e. with a voltmeter in parallel with the motor. I guess this means that there is too much resistance in the motor and not enough current flowing to operate what ever 'holds in' the connection within the black box once the switch is released. Any other suggestions? I think I'll just live with it for now, unless it starts blowing fuses on a regular basis.