Most flats, and some houses, have a leasehold, rather than a freehold, tenure. A property occupied under a lease is rented, rather than owned outright, for a specific time, typically 125 years on inception.
As each year passes the right to occupy the property shortens in time so, for example, a lease established 40 years ago would now have just 85 years before expiration. A lease is a deteriorating asset which, if left unaltered, will make it difficult for the owner to sell and problematic for a prospective purchaser to raise a mortgage.
A qualifying tenant (leaseholder) of a house may have the statutory right to extend a lease or to buy the freehold.
Qualifying tenants of flats have some additional rights including that of reducing the rent to a peppercorn rent, though a lump sum will be payable to the landlord in return and to add a further 90 years to their lease term.
You may find it worthwhile to approach the landlord to extend the lease even if you have only recently purchased the property.