DHL Express lost my passport, how do I hold them accountable?

I recently applied for a visa from BEDRIJF A (UK visa application center) and requested a delivery service via BEDRIJF B to ship my passport back. However, I did not recieve my parcel when I got the delivery notification from them.

The courier insisted that they have delivered the parcel (while providing a signature that is not mine) but I am sure they did not check for personal ID even if they did. Of course none of the residents at our aparment building would take my package by mistake.

It has been over two weeks since I lost my passport. The DHL Express customer service told me they have started investigation over a week ago. The progress is very slow, as the shipper (TLScontact) does not have a phone number so I have to wait for their slow email response. I want to hold BEDRIJF B accountable for their mistake and cover all the cost that involved in recovering the passport and issuing a new one. However, the only access to BEDRIJF B I have is through their frontline and via email. The staff has not been helpful and each email exchange takes more than one work day. What should I do?

Thanks for your help in advance.