Perhaps the time to think about dealing with the Vet and the Crematorium Service would be best left until your grief has lifted it's heavy hand from your heart somewhat.
I understand the urge to sue but I'm not sure that will ease your pain. Obviously you do not want to see this happen to anyone else. You may wish to consider meeting with all parties present to determine what formal protocol changes your Vet and the service have already taken or will take in how they "do business" to ensure this unfortunate situation is never repeated. If they fail to respond, then may be the time to involve the courts.