Finally a chap from Mercedes-Benz Finance called and they've waived the whole charge for damage and have even reduced the extra mileage charge. Cool.
The called said they have completely revised their program for the return of cars in the PCP or leased plan. They've outsourced it to another company in order to provide an objective service, and they have reduced the stringency of car analysis. Critically - he accepted that extra stringency cannot be added after the signature event. Now customers are required to read the return standards prior to the final sealing signature. They've also stopped this close analysis of special lamps which conveys the idea of 'trying very hard' to find problems.
I'd like to think that this blog has caused this - but he also said that other people have complained too - so I guess that shows the value of complaining.