Update:
Just did another different Lupo with exactly the same symptoms. All you can hear is the relay clicking, but nothing happens when you press the window switches.
Another easier solution...
Once I removed the slider/cable/rail/motor assembly thing, I pulled the motor out, separating that plastic drive spline from the cable drum (that pulls the window up or down). I thought, "I wonder what will happen if I plug it in now, without the cable assmbly attached to it?". It worked. Yes, the motor worked perfectly. Up, down, start, stop. Perfect.
Must be jammed tracks/cable? Assembled it again, and the damn thing carried on working as if nothing had happened. Very odd. Put it back in the door, this time with glass clamped down. It worked again. Normally. Not slow or stiff. Normal.
Back out again. I needed to know why. Then I noticed that the plastic spline gear thing that the motor rotor drives, was slightly warped. The central spline was running at an angle, presumably where it was held shut for ages, as most of the time the passenger window is never used. The only reason it worked again is that when I reassembled it, the splines were in a different position (by pure chance), so there was no angled stress.
Then it hit me. The slight spline shaft angle acts a lock, preventing the cable drum from turning once there's some tension on it- which there will be once the motor drives the window hard up against its stop (at the top).
Solution: Pull everything out of the car as before. Separate the motor from the cable drum. Don't move the window sliders at all. Mark the white plastic splined drive cog (that I've marked 'up' and 'down' in the pic). Run the motor so the plastic worm gear spline drive is 180 degrees out before reassembly. All sorted. The new position should, over the next 20 years mean the thing will slowly unwarp... It should carry on working perfectly now, asssuming the rest of the system holds out. Yay!