I used DHL to ship a parcel to the United Kingdom

I used DHL to ship a parcel to the United Kingdom. The parcel has been stuck in the status "on hold for more than 8 days, no call from DHL until I called to ask why it is in that status. I was advised that I failed to provide the necessary documentation, said documentation was extremely vague and attributed to requiring customs clearance to leave the country (I assume). When I requested for them to return my parcel to me, they effectively are trying to get out not returning my full fee for their failures in not collecting the right documentation and not communicating with me. I will not be using them again. As of today, my parcel is still with DHL (not sure whether it's in Cape Town or Johannesburg) and they have not responded to my email asking for the return of my goods. They are holding my parcel ransom for their failures.