Kenwood Chef A701a planetary not working?

I have a Kenwood Chef A701a, running fine until a few days ago, midway through kneading dough with the dough hook it stopped working properly all of a sudden. It still turns but the "planetary" action part of it doesn't work - the dough hook just spins freely in the socket which makes it useless.

I have open up the "chrome dome" and the teeth and planetary gear all seem ok. What else might cause this problem?

Also does anyone know where I can get a service manual for this machine?

Thank you :)