I have just received a letter for a previous occupant for the flat.
This is fine and happens quite often. I return them with the information 'Addressee Gone Away. Please Return To Sender.'
But I can see very clearly through the window the words 'NOTIFICATION OF LEGAL...'
I have lived here for nearly 12 months now and have had to send back one of these before. I suspect it's the same debt-recovery agency.
What do I do? What if a bailiff turns up?
If you're relying on the the Post Office to return to sender, hh, don't count on that process being completed any time soon unless the return address is visible on the outside of the envelope. Letters that have to be opened to establish the sender's identity or address can remain for months with the the PO department dedicated to such matters. I suspect even the ones with the sender's address on the envelope are a pretty low priority.
If the sender's address is accessible it is probably a good idea to look them up and give them a ring - though this is by no means your responsibility. I wouldn't lose any sleep worrying about bailiffs turning up. Real bailiffs - the ones instructed by the courts - are, in my experience, neither stupid nor aggressive. They'll probably do enough research to discover that the debtor no longer lives at your address but in the unlikely event they do turn up they will quickly realise you're not the person they want and will report back to that effect.
A more likely problem than bailiffs is that court judgements against the address could affect your credit rating. If you find you're being refused accounts you would normally expect to be a formality it might be worth checking this out.