Opening the post

[tinyjo]

So, I'm wondering. Most of the mail which arrives at the house for old occupants, I either re-direct or return to sender. I'm getting one or two letters a week though, from business sources (they're franked, not stamped) with no return address. These days, the Royal Mail will just throw these away if I stick them back in the system. So am I allowed to open the letters with the purpose of getting the sender's address to return the mail?