tenant changed the lock without asking

it never stops with these tenants - i am so supremely glad they are leaving in a month. Tenant came back and mislaid his bottom Chubb key for the front door so instead of phoning the emergency line at the estate agent, he just....changed the lock. AND IS NOW TRYING TO CHARGE ME FOR IT!

I did an inventory check in, so literally have photographic proof he received multiple sets of keys with the Chubb key attached, PLUS he has breached his clause in the contract which explicitly states he cannot change the lock without asking the owner.

The estate agent is trying to sort - since i am in the US but i'm seriously concerned as now no-one has a key to get in - not me, not the estate agent. Obviously they will return the keys when they leave, but we need to get in to do viewings which they also are obligated to provide access.