"It is only an offence if you open someone else’s mail ‘without reasonable excuse’ or if you ‘intend to act to another’s detriment'.
"..if you are receiving bank statements/cards in someone else’s name then you should act on that immediately. You should tell the sender, either by returning it marked “not known at this address” or by opening the mail and calling any number provided within."
After my OH's cousin moved out of here we started receiving debt collectors letters. After a few months I started opening them and phoning the companies. They stopped arriving and no one questioned me about why I'd opened the letters.