Tenant has pets without the landlords permission

Can any readers provide any advice on what to do when a tenant has pets without the landlords permission please?

Scenario: my new tenant moved in just over two months ago along with a ferrett, two cats and a dog. I was not aware that the tenant had pets, I would have said no and would certainly not have granted permission.

We only found this out when we visited the property at the tenants request to fix problems with the kitchen taps.

We immrediately informed the letting agents about the pets and they admitted that they had no idea that the tenant had pets and had no notification from the tenants that they would be bringing pets either. The tenancy agreements specifies no pets.

The letting agents then sent the tenants a letter (we had to ask for a copy) informing them they were made aware of pets in the house and that they will be liable for any damage deductable from their security deposit.

After a long debate as to whether to serve a section 8 notice on the tenant or wait until tenancy renewal is due to give them notice to leave we have decided to ask the agent for a £150 top up security deposit

The agent has not responded.

We would appreciate comments or advise from with ways forward?

Thanks

Russell