Unlikely that he's taken to health biscuits and doing good among the poor from what you've said. My suspicion is he's found an angle where he charges you for something and you'll need to prove you aren't liable for it. To prove it may entail more time and expense than the sum he claims.
Sounds like you're well covered. I have heard of landlords who routinely send a bill on the chance it will be paid without question, just assumed to be legitimate. That doesn't explain the address part. Some areas have a "renter's rights" group, maybe check into that.
Maybe the one you are dealing with is not the former landlord? Maybe they are are a relative who has inherited the house. Or maybe the old landlord sold it to another..
Well he may have been having a breakdown during those time and now has come out of it and decided to set things right? Who knows. :)