Water supply pipes, who owns what?

[saiello2061]

Hi,
I live in an upstairs flat. A pipe burst inside of the flat below. The supply to the flat below tees off immediately after a stopcock under my floorboards. If I turn the stopcock off, its turns off supply to both flats. My question is: who is responsible for the burst pipe, me or the owner of the flat downstairs? I own the leasehold of the flat below.

Many thanks 😊