To think we are stuck with hideous neighbour.

I don’t know why some people are so critical of you finding it extremely hard to live beneath her. It sounds like a nightmare.
Also some of the behaviours you describe, plus the fact that her father has possibly given up on her, make me wonder how much is truly mental health and how much has a behavioural element. Something like borderline personality disorder can absolutely be improved IF the person is willing to engage with treatment services.
I wish you all the best. It sounds truly crappy.

Doesn’t apply to a direct complaint to the neighbour. It only needs to be disclosed if the complaint has been escalated to the local authority.

I don’t understand why you can’t sell?

I think your only option at the moment is to get a builder round and discuss the cost of sound proofing your flat. Get different quotes.
It would be worth taking out a mortgage or loan to do it as you can't sell it with this shenanigans as it is.
Other posters telling you you should feel sorry for this woman beggars belief. It would drive me to drink. She has relatives and you are not responsible for her or trained to deal with this.

Also this:

asbhelp.co.uk

Oh My -some people on here are SOO JUDGEMENTAL! If they are soo concerned about Yr use of 'offensive language' when this person is putting you thro hell-may be they should hv her as THIER NIEGHBOUR! & sèe how they feel,then! SOO EASY to judge YOU from thier cosy non nightmare situation homes! When you hv a nighbour like that you get frazzled & worn down .My heart goes out to you. I worked at a support charity for victims of crime and had many clients who had antisocial nightmare nieghbours & had one myself upstairs- its like TORTURE! and it's not uncommon for a family to buy somewhere to wash thier hands of a problem relative. It maybe worth contacting citizens advice. They will,tell u to record everything u can both audio & writing dates and times of events. Its laborious- but just coz someone has mental health problems doesn't mean they hv a right to harass & threaten you. It's harder because it's a privately owned property she doesn't rent so can't evicted. But if you have recordings etc the police should take it seriously. I wish you luck with it. Its a really horrible situation be in. Many folk with severe mental health issues can manipulative and behave calmly when authorities come round. She no doubt has a personality disorder, in which case the therapy doesn't work- as well as other issues., the problem is mental health services will not get involved unless she requests it / help for herself or is deemed a serious physical threat to herself or others, and even then they can only commit a person for a short period of time. & it certainly used to be the case that if you sell a,property with a troublesom neighbour you had to tell the perspective buyer by law. So you are in a really hard situation, I send you good wishes empathy and hope something happens to resolve the situation all round. It speaks volume the father doesn't want to talk to you about it & doesn't visit. But the police should take harassment and threats seriously if you have enough proof. But even then folk with that kind of severe mental health issues don't tend to change. The only other thing I can think of is she may have been precribed medication which helps,her but she is not taking. That's v common.

This is quite useful:

www.ageuk.org.uk/information-advice/care/housing-options/home-safety/neighbour-disputes/

You need to first collect all your evidence then decide which options to pursue.

If I were you I’d be logging and recording the details of every incident for eg two weeks, then taking everything to the police. I guess question is whether there is anything criminal or capacity related that you can pursue. The fact they’ve had so many calls already will be logged. If you can, try not to let her see you’re recording everything, but essentially you want enough evidence to make it impossible for anyone to ignore.

I'm in the US and here you could sue her father for interference with peaceful enjoyment of your property. If she can't or won't be a reasonable neighbor she needs to control herself or be moved.

Here in the US you could get an enterprising attorney for a filing fee.

She is effectively stalking and harassing you when looking through your window and threatening you.
keep a diary maybe a cctv camera and every time she directly harasses you report to the police for that and or stalking. There’s tougher laws on that aspect. She will be warned and if she does it again and again then should be arrested for it. Maybe then daddy will give a damn and or she eventually gets a prison sentence.

It's brutal, @Neighboursfromhell - my friend lived in a lovely housing association garden flat and had a very similar issue with a rich daddy buying an upper flat {1870's house} for a mentally unwell adult daughter.

It was really unpleasant for her to live with that - bulimic {I can't begin to describe the grotesque blocked drains}
I never saw the woman but she affectedly friend badly due to the drains {which flooded by friend's front door/courtyard} and noise that she made.

Flats are hellish if one has bad neighbours, especially above you.

I don 't know what to suggest- But she could live there for decades yet.

I'd maybe sell up, as it would be very difficult to live with someone so antisocial.

Detached houses are bliss.

{I don't live Iive in one!}

First poster replying put the boot in. Others felt emboldened to follow that lead.

I'm now waiting for some genius to suggest OP "gifts" her flat to the "unwell" neighbour...

Not sure why you're getting such a hard time OP, your mental health will no doubt suffer soon living with such a terrible neighbour. No one would be ok with this situation. I would continue to pursue all avenues, you have a right to a peaceful home. Don't give up

Keep a careful record of this behaviour and report threats to the police (even if they take no notice). The criminal offence of harassment includes alarming a person or causing them distress, but it has to happen at least twice, to comprise an offence.

In every lease for a flat I've ever seen, there is a clause about not causing a nuisance. Do you have a separate freeholder for the building?

@Neighboursfromhell "
B) it makes a difference in terms of options available as technically she isn't a tenant, nor is she the home owner."

It makes no difference. So, in this case it looks like daddy is the leaseholder. If he installs a tenant (or loved one) who causes a nuisance, daddy is liable.

One is obliged to report neighbour complaints on a seller form. If OP has made a direct complaint, she'd be obliged to reveal this on the seller form.

I am sorry to hear this op. Be very very careful re police. It is easier to charge you with a hate crime than help your neighbour. I'd stop reporting, cut your losses and leave. Your life is made of time, not money. You cannot waste your life miserable.

If she's threatening you you can apply for a non-molestation order online, you don't need a solicitor

Might not want it right now. Might want to pay a derisory amount etc

Op if the father wants your inherited flat then why don't you just sell it to him and move elsewhere?

Have you tried selling?

@ChildlessCatLadiesRuleOK indeed. Even in units for people with very significant mental health conditions, if someone was racially abusive, sexually or physically abusive, it was reported to the police every single time and their capacity was assessed. They would be charged if found to have capacity.
If she is living independently without support then it could be unlikely that she will be assessed as lacking capacity to harass and threaten you. Therefore she should be treated by the police as anyone else would be. Even if she didn't, for example if her treatment of you arose from a delusional belief, then the police still need to be aware of this. They might know something you don't, and so this could be vital information to provide.
The rest of the stuff about whether she is the home owner is really not relevant, it's unlikely her dad will evict her is it?

That doesn't sound very invested if that's how much he's visiting. Does she have carers who visit? If not, it sounds like she needs it.

If the end goal is to acquire your inherited property, can you offer to sell it to him? That depends how much you actually want to stay there, of course.

The OP mentioned the ownership/inheritance of the flat. Not sure what difference the flat being an 'expensive inherited' one has re the issue if the mentally ill neighbour. As pp stated the OP can still sell up and move.