Common causes
Sealant between the frame and the wall has aged or cracked.
Drain holes in the sill or sliding track are blocked with dirt.
Sash gaskets have hardened and no longer seal.
A warped sash or sagging hinge stops the sash sitting flush.
What you can do yourself
Clear dust and debris from tracks and drain holes.
Note where and in what weather it leaks, to help the contractor diagnose it.
Never climb outside or work on the external wall at height yourself.
When to call a member firm
Resealing, replacing gaskets or adjusting the hinges fixes most leaks; a badly damaged frame means replacing the window. If the leak involves the external wall or common parts, also tell the management office.
Official sources
This page is general guidance, not legal advice. If rules change, the government department's information prevails.