Breach of tenancy agreement (pets)

Other

Hi
I have been made aware that my current tenants may have recently bought a puppy. I am waiting to find out if this is true, but, if it is, then they are in breach of their tenancy agreement, which clearly states no pets without permission from me.
They asked me if they could get a dog around Christmas time, via WhatsApp messages, so it is all documented.
I said they could but only if they met certain conditions such as an additional deposit and a new tenancy agreement and inventory, all of which had to be arranged and paid for by them.
They agreed not to go ahead with the dog, but it looks like they may have gone and got the dog anyway.
I%E2%80%99m not sure what I should do initially, once I have confirmed whether or not they have the dog.
Thanks