Hello. Is their flat owned by the same freeholder please?
Thanks. Adverse possession by a tenant can only benefit their freeolder, and as this land is already owned by the freeholder, it can't apply. I suggest you show the land registry plans to your neighbour and say the land is part of your leasehold interest. Also write to the freeholder, as they will need the freeholders permission for the extension, and you can point out that part of the land in question is in your lease.
Hi. No, it shouldn't make a difference because AP can't apply and there has been no formal amendments to your lease to remove this land, which can only be done by deed.
No problem. Please leave a rating for my help. Best, WB.