Julian,
This doesn’t make sense. The machine uses an air pressure water level control system, so theoretically it can’t overfill, even if it reaches a drain sequence, followed by a refill. Even if it did, this would flood the undertray and activate the flood protection system.
Therefore, the machine must be losing water through the door seal, or the level control is faulty.
You say that you have had someone to look at the machine; frankly what he has told you cannot be true. I would suggest that a check in the Directories for a UKW engineer might be your best option, either that or send for Bosch. They do know their stuff, and it would certainly be cheaper than replacing what is a good quality appliance.
Regards,
Penguin45.