I loosened off the latch and squirted a good dose of lithium grease into the mechanism behind. Since I didn't have direct access to the area behind the latch, I was careful not to remove the two screws completely in case anything dropped down behind - someone mentioned that the screws also held the motor in place. I also took the opportunity to p;lay around with the lateral position of the latch to minimise the force needed to close the door. It's been several months now and the door continues to open and close very sweetly! Can't believe VW wanted to charge me around £1k just to investigate this issue.