To ahmskunnirt and fredpuli47.
I suppose I did go over the top in how serious it is but it is a indictable offence and always has been. Whilst it is no where near as serious an offence as it used to be it is still a serious offence as offences go (as I said it always used to be a automatic custodial sentence).
You quoted s 84(3) of the Postal Services Act 2000. IWhilst this may apply if I were to act as then landlords lawyer I woul;d argue that it doesnot. As you pinted out its difficult to argue that the landlord is intending to act to a person's detriment. And also, the mail was not "incorrectly delivered to him". The mail was delivered to the correct address, and the landlord picked it up. Not much of a distinction I know, but enough for a lawyer.
What is appropriate here (I've seen someone been found guilty of it in court) is s1(2) of the Regulation of Investigatory Powers Act 2000. This act deals with powers of the Security Services and s1 covers "Unlawful interception". This covers post and "interception" covers the opening of said post.
I must admit that the sentence is not as severe as I thought (2 years max), s1(7)(A). But I still hold my point that it is (or at least was) a serious offence. I'll try and find the pre-2000 law on the matter as I'm pretty sure that it used to be a much more severe sentence.