Hi, Just an update on this, having been backward and forward with the supplying dealer to no avail, I got the car booked into a local German car specialist (that helpfully is only 5 minutes walk from my new job)
They did a thorough health check on the car and found the driver side top mount was worn, and also the Battery was at 24% (is the original battery and now 7 years old) - both jobs have been done and there seems to be no knocking now....