If you are also the freeholder then it affords you a number of advantages.
As freeholder it would be for you to permit any request by the lessee of the other flat the right of extending their lease to 999 years. You would be able to charge a fee for this. The reality is that lenders will usually lend on a leasehold property where they is 70 years remaining on the lease so any purchaser of the above flat would be able to purchase for another 20-30 years without needing to extend.
Similarly, if in future you required to vary the terms of the leases for whatever reasons (eg. modification of a covenant contained within the Lease) then you would need the consent of the landlord, which would be you if you bought the freehold. If it wasn't then it would be for the current landlord to decide whether they wished to charge a fee for this, and there may also be problems getting in touch with the landlord so there is also a practical advantage to be had.
You would also have the right to receive the ground rent payable by the other lessee.
It also affords advantages in enforcing covenants/rights against the other lessee in that you would not need to make the current landlord a party to proceedings.
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There are not a great deal of disadvantages, save that the lease to the top floor flat will contain lessor's covenants in the same way that yours does - it will be your responsibility to comply with these if you become the freeholder.
I cannot really say, you would have to speak with the agent about the value of it as they will have knowledge of the local area. But it will probably be thousands, rather than tens of thousands. You will certainly be able to negotiate with the current landlord on the price as well. Chances are that he'll be quite grateful to get rid.
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