Hi i'm afraid you cannot carry on letting the apartment to those tenants if they orginally came to rent the flat through foxtons - to be fair this is the same for every single agents - most tenancy agreements that both you and the tenant sign do have this in the small print whatever the agency you use..  . if they stay renting and you are found out they will sue for commison they are owed - i know this as i had a similar situation myself.

Its up to you whether you want to take the risk, but they are ruthless with this kind on thing.

Hope that helps