In your pursuit for a resolution to this problem, and by opening a communication addressed to a third party, you are (inadvertently) in breach of the Postal Services Act 2000, Section 84.
http://www.legislation.gov.uk/ukpga/2000/26/part/V/crossheading/offences-of-interfering-with-the-mai...
84 Interfering with the mail: general.
(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.
Whether you have acted to the detriment of and without reasonable excuse is open to interpretation, but in future you should really err on the side of caution.
The guidance from Royal Mail is "If you receive mail intended for another person, such as a previous owner, it should be returned by marking the envelope No Longer at this Address, please Return to Sender" and placing it in any postbox.
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Only mark a post as helpful if your issue has been resolved.