Personally I wouldn't open anything not addressed to me and as dzug suggests just stick it back in a post box with return to sender not at this address on it.
However opening somebody else's mail is not strictly illegal, definitely not a black and white matter. If it has been addressed to your property and by opening the letter you are not acting in a manner to do damage to the person then you have done nothing wrong. However as suggest I'd err on the side of caution and just send it back.
I'm in a similar situation to yourself, we just mark it and send it back, we have had the odd balif but they are fine if you explain they no longer live there. Make sure these people are no longer registered at the address and that you are, that should help deal with any problems in the future.