Kevman I think the inspector has a pretty good idea how the majority of the post went missing, hence the comment that it was irretrievable, but without proof and evidence of exactly where when and by whom, combined with difficulty of any employer tracking down 'bad eggs' there was little more to be done, save to moniter and tighten the system from now on.
The volcano must be to blame for some current delays, but do note Anita's suggestion to continue to report missing mail.