84. - (1) A person commits an offence if, without reasonable excuse, he-
(a) intentionally delays or opens a postal packet in the course of its transmission by post, or
(b) intentionally opens a mail-bag.
(2) Subsections (2) to (5) of section 83 apply to subsection (1) above as they apply to subsection (1) of that section.
(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.
(4) Subsections (2) and (3) of section 83 (so far as they relate to the opening of postal packets) apply to subsection (3) above as they apply to subsection (1) of that section.
(5) A person who commits an offence under subsection (1) or (3) shall be liable on summary conviction to a fine not exceeding level 5 on the standard scale or to imprisonment for a term not exceeding six months or to both.
http://www.opsi.gov.uk/acts/acts2000/00026--f. htm#84