next year student tenancy agreement signed but now 1 wants to drop out

Tenancy Types and Management

our agent has 6 students signed up for a tenancy starting in july 2013. 1 has decided they can't afford it so 5 have asked to renegotiate the rent for 5 occupiers meaning a loss of about £2k in the year. We are trying to sell this property to an investor. is there any redress available to us? should we just let them go and ask agent to find new tenants? otherwise, the value will drop substantially as unlet. some advice is being obtained from our solicitor but i fear we can't hold them to a contract which they can give reasonable notice on ending before performance actually begins. would very much welcome some advice.