The position of the law regarding tenancy matter is that
The tenant is responsible for internal repairs
Regardless of other points you made
It does not relate with this issue
It is your responsibility as a Tenant to fix it.
Don’t even think you will fix it and break it back when you are going
As at the time yiu rented the apartment, the WC was in good condition and you must leave it in good condition else,
Your landlord can SU£ you for malicious damage
Ignorance of the law is no excuse