FHMonkey
Many, many years ago (before e-mail was invented!) I experienced missing post over a period of 3 months or so. I was concerned about missing bills, then missing the reminders and then getting into more serious trouble.
Since you have a record of the e-mails, then prepare a list of everything that you know is missing, with as much info as possible, date of posting etc. Find out the manager's name at F Hill DO, and write to him/her personally.
I had a very useful conversation on the phone with the manager at that time, and was promised that all items of missing post would be investigated, and was thanked for the time and trouble I had taken to report it. It seems that sometimes there's a rogue member of staff who likes to pocket the odd letter, and the penalties for stealing items of mail are very severe. I think one or two items did come to light, and I received a load of first class stamps as some compensation!