Who is repsonsible for maintanence?

I couldn't open the door to my flat, a large sliding door, so called the property manager. He couldn't open the door, so called a lock smith/glazier, who came over and removed a window pane to gain access. He discovered that an old locking mechanism at the bottom of the door had engaged. This is something that we have never used as the door locks with a key. I have disputed this charge with the property manager, and was awaiting his response. But today received an account and demand for payment. My dispute is that the old locking mechanism can only be locked from the inside. And when it is locked you can't open the door. So I can't have locked it and open the door to get out! I don't believe I should be held responsible for this old lock engaging. The glazier himself said it was very loose. I think it just fell and locked when I shut (not slammed) the door that morning. This is about an old lock failing - due to age, approx 30 years, and is the owners responsibility, not mine.