Mine did the same when the car was still under warranty and it got repaired FOC.  A couple of years later, not long after the car was out of warranty the same seat broke again.  A quote from VW to get it fixed was £10 for the part and £150 for the labour.  I'm not paying that much so have decided to put up with the minor inconvenience of  it not sliding.  I rarely have anyone in the back and if I do I just ask them to get in the passenger side.