whos responsible for pipes in a concrete floor of purpose built block

Maintenance

We have a flat in a purpose built block, the floors are concrete and the pipes that take the water from the water main to the bathroom, kitchen etc run through the concrete floor. The flat below ours has had an ongoing leak through the ceiling of their kitchen, our kitchen is directly above.

The pipes in our kitchen are leak free, so there is a chance that the leak is coming from the pipes in the concrete floor. Whos responsibility are they, we are a leaseholder, I assumed the freeholder was responsible but they claim not to be?

Any advice would be great.

Thanks
Neale