Defence? You have no defence to make...
Just how does the landlord intend to show that you caused this problem I wonder? Was it when you actually walked on the floor or was it that night you stupidly brought the elephant home from the zoo?
The problem belongs to the Landlord - write a formal letter to him asking him nicely to get it repaired and keep a copy of the letter - it is not up to you to explain how the fault may have happened - you are merely a tenant not an expert in plumbing or electrical work or boiler maintenance or floor laying or house maintenance - you pay him rent to do these things on your behalf.
After a month write him a second letter asking when he will see to the work and contact your local council - they will have a department who can advise and even intervene.
You do not really need to know why or how this has happened - that's for the Landlord to discover.
Peccavi
March 2009