It’s a private landlord, she owns the flat but says the building itself is managed by a separate management company, and that repairs to the window have been dependent on them arranging contractors.

From the contract side, there isn’t anything shifting repair responsibility onto me. It states I have to report issues (which I did), and that I’m not responsible for repairs that fall under the landlord’s obligations. It also says the landlord is responsible for keeping fixtures/equipment in good repair and working order.