"s.84 refers to intentionally opening a mail bag or opens a postal packet in the course of its transmission by post. "
I don’t think it does, hc. Section 84 (3) deals specifically with items that have been delivered (and are thus no longer in the course of transmission by post):
84 Interfering with the mail: general.
(3) A person commits an offence if, intending to act to a person’s detriment and without reasonable excuse, he opens a postal packet which he knows or reasonably suspects has been incorrectly delivered to him.
So he has to know or reasonably suspect it has been incorrectly delivered to him (fairly easy if his name is not on the outside) but he also has to intend to do so “to the detriment” of the addressee (or the sender) and without reasonable excuse.
So, if you receive a packet addressed to somebody else and you know it contains something valuable and you nick it I think you are banged to rights. If you open a packet to see who it is from (to either protect your own interests or the sender’s or the rightful addressee’s) you have a reasonable excuse.