Long post - sorry!
We bought an apartment a couple of months back. It is an older property divided into two flats. We have the upstairs one. The owner downstairs X owns a building repair company and rents his flat out to one of his workers. He has owned the place for 2-3 years.
A couple of weeks back I got a knock on my door. X had some problem with water blockage downstairs. Apparently water is backing up in the kitchen downstairs and not draining properly. I told him to speak with DH regarding it. But also told him that if he wanted to charge for anything as communal repairs, he would need to send us an invoice first for us to agree on, and we would prefer a neutral third party company to do the work.
A day later, some guys come and did the work of pressure cleaning the drain and repairing an iron pipe that had cracked a bit with age.
A week later X knocks again on my door, and said that the tenant below was on holiday and the apartment below was flooded as his kitchen sink had backed up with water, and we musn't throw cooking oil/lard down the sink. We never deepfry anything. We are vegetarian and pretty much bake everything. And in any case, even if we did use oil, we would never throw it down the sink. So I was quite offended and told him that we know how to manage our apartment and we are not stupid to throw stuff down the sink. I an Asian, and I felt that he thought I was lying and frying pakoras all the time 
.
DH contacted X and said that if the drains downstairs had backed up again, it really has nothing to do with us, and the repair guys didn't do a proper job. It seems all was ok.
Well, today we got a bill from X for £300 as as our share of the drainage work and the changing of one pipe on an outside wall. In that letter, was a long explanation on how his kitchen was damaged. I called him up and said that the bill was too high and that a) the drainage blockage in his apartment was really not our issue b) I was not comfortable with him using his own company workers, we would have preferred a neutral third party quote c) I would be willing to share the material cost of replacing the split pipe, but that would be it.
X got quite aggressive on the phone and saying he had done us a favour and we now didn't want to pay and that I was insulting him. I told him I did not want him to raise his voice with me and that in future I did not want him knocking on my door and that he should set up a time to come by when DH is around to resolve any communal issue. And I put down the phone.
Guess what, 20 minutes later, a knock on my door and there X was, asking for money and generally being an arse. I told him that I had told him not to come by when I am alone at the home, and next time I would call the police.
I am usually quite calm but this guy really riles me up. He is aggressive, a bit sneering and generally seems a bit of a loose cannon. Also, I work from home and do not want to be disturbed without prior notice, except if there is an emergency.
So,
AIBU to have said I will call the police if he makes a nuisance of himself again?
AIBU to not want to pay £300 for something that I am not clear is a communal issue?
AIBU to not give a rat's arse about the condition of the kitchen below and think that it is really not my problem?