Your big advantage is that they have actually seen the fault. I had a similar situation with a Berlingo some yrs ago. Every so often the driver's window would not drop. Turn off engine, start it again & it worked ok. No fault code showing & main dealer said they could do nothing if they could not see the problem. One day it happened right near to garage. I drove it round there, left engine running & got service bloke out to see it.
I suggested they replaced motor & switch under warranty which they did & the fault never occurred again. As I said you need to put your foot down & insist they replace parts.
Try emailing VW UK & even VW factory in Wolfsburg. I find this sort of thing usually works, make enough noise & it usually brings results.
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