I had a letter addressed to my address, house number, post code and everything, but with someone else's name on it. I've lived here for over 10 years and have never heard of the persons name in connection with my land lord or any previous tenant.
I opened it, read it (was a greeting from some other individual saying how their visit was nice etc etc) and it had 20 quid inside.
I know all of my neighbours names, so it wasn't one of them addressed by mistake to my house.
I kept the 20 quid.
True story...
Incidentally, I'd not bother about wreaking your revenge over the partially opened post unless this happens repeatedly with all of the post addressed to you.
You say you have stuff from the Mod? Careful now... if you pose a security risk, they might have you whacked.