on the 19.6.16 3 tenants signed a general tenancy to rent my home from 15.7

I gave notice to my current tenant which was coming up for renewal on a fixed term -The new tenant now say that they have a lease on their current rental property until Oct and will lose their bond if they break their lease . Can they change their mind after signing the lease - what is the legal wording or extract that I can use in a letter making them aware of their legal obligations? Thanks for your help

What can I practically / legally claim from them ?