Adverse possession attempt

Hello, I am seeking some guidance on an attempt to transfer the title of our basement please. It is rather complicated I'm afraid.

My husband & I purchased our flat (F1) in April 2024. It is leasehold and the title deeds show ground, first and basement areas for F1 plus a private garden as being ours.
The basement is mostly underneath the common parts of the building and another ground floor flat, F3. Subject to a proper survey we believe a small portion may be under our boot room, if not then it is certainly adjacent. The current access is via a trapdoor in a different ground floor flat (F3) but we could get into it from F1 with some excavation from our boot room. We believe this was the idea when the property was converted from a single dwelling but it wasn't completed. However it is clear that the title deeds were set up to include the basement in F1.
The neighbour in F3 has advised us that she ("DF") will be applying for a transfer of the basement to her title on the the grounds of Adverse Possession.

There is a Freeholder for the property, a Management Company which holds the head lease and then 6 flats with sub leases including F1 & F3. My husband, DF & one other flat owner are the directors of the management company.

Brief history of the salient facts:

1998 conversion of single house into flats and leases granted

November 2000 F1 purchased by "PB"

March 2013 PB died intestate and squatter/s moved into F1

April 2013 F3 purchased by "P&P"

At some point in the next 5 years P&P took up the carpet in their hall and discovered the entrance to the basement. At this point they installed a trapdoor but we do not know if they ever used it.

February 2017 P&P extended their lease but made no mention of the basement (we do not know if this is before or after the floor works)

July 2018 DF purchased F3 from P&P. The sale particulars made no mention of a basement.

Q1 2022 Bailiffs evicted the squatter/s from F1 as a repossession on behalf of Bank of Scotland

February 2024 we exchanged on F1. The title deeds show the basement but, due to the state of the property and the rubbish left in it (which we cannot touch until completion), we cannot tell if there is direct access from F1.

April 2024 we completed on the purchase of F1.

Over the course of the clearance and renovation during the next few months we discovered there is a hole in our boot room floor that could facilitate access to the basement but it is full of builders rubble. Since we had a lot of work to do to make the rest of the property habitable we decided to leave this work for a future date. 

In the meantime we met DF and visited her in F3 on a fair number of occasions. We told her we have a basement but no current access. She showed us the trapdoor which leads to the basement and said that although she has access she never goes down there because it is dark, dirty, full of spiders & probably damp and in need of tanking. At first she did not try to claim it is hers, just acknowledged it is there. Further, she has always had a very large chest of drawers over the top of the trapdoor which she cannot move by herself (she lives alone) making access impossible without help. She promised to show us the basement "at some point". It was not clear from her description if there is just our space or additional space that might belong to someone else. Over the course of the next few months we visited her regularly and the chest was always in place blocking the trapdoor.

In the early summer of 2025 DF started talking about "her" basement. She asked our builder to have a look at it and together they moved the chest off the trapdoor. He has told us that when he looked in it did not look like anyone had been down there is a long time because there was a huge build up of dirt & cobwebs everywhere which were undisturbed.

5 July 2025 - as part of a different matter I sent DF an email which included a copy of our title plan. Since it also showed our basement I asked her to dig out her paperwork so we could clear up any discrepancy.

1 August 2025 - we received a letter from lawyers acting for DF acknowledging that the title to the basement is solely on the deeds of F1, there is nothing on F3. They suggested this is a mistake and should be changed.
Later that day we suggested to DF vis WhatsApp that things should be resolved amicably and requested to see the basement to which she agreed.

6 August 2025 - we visited F3, the chest had been moved so we could access and view the basement. There was a suitcase and a couple of boxes in one corner, otherwise it was empty.

27 August 2025 - we replied to the lawyers that we do not think the inclusion of the basement in the title deeds for F1 was a mistake and that we intend to keep it 

29 September 2025 - we received another letter from DF's lawyers advising us that she intends to apply to the Land Registry to have the basement moved to her title on the grounds of Adverse Possession.

Questions:
Do you think DF has a case for Adverse Possession?
Does the eviction of the squatter/s reset the clock on any attempted claim?
Does the occupation have to be "open and notorious" and if so how can it be when this is about a basement and the owner is dead?

Please do let me know if you need any further information
Many thanks
Sue

Hello Sue - We cannot tell you if DF has a case for Adverse Possession until an application is submitted to us accompanied by the relevant evidence.  Our decision would be based on the evidence presented to us and the additional checks we carry out after receiving the application. 

I would suggest you seek legal advice on the matter.