Tenants leaving early

Tenancy Types and Management

I have just completed on a house purchase where the tenants were in situ - 4 in all with a joint and several AST. They signed a new AST with us which runs until August. Two of the tenants are now saying they are leaving at the end of May. We have let the property to a new group of 4 from the beginning of September (students) who do not need to start their tenancy any earlier. Our AST states they pay the rent until a new tenant moves in.
We are now left with a shortfall of £860.00pcm (between them.) How do I start telling them of their obligations etc. We have their deposit monies of £400 each. They have not yet paid us any rent as it's due mid-month.
I don't want to get involved with solicitors at this stage as it will be so costly.

Thanks for your help.