RLA Tenancy Agreement

Other

In the past 6 months I have had two tenants with the RLA Assured six month tenancy agreement. I explained to them that it was a legal document and the tenancy was for six months. They both agreed. The first tenant left without telling me and the second tenant gave me just one week's notice. When I mentioned that he had signed an agreement for six months his reply was 'too bad, I am leaving'. He left after one week. I consulted a solicitor, who advised me that there was nothing I could do about this. I now have to have the flat cleaned and advertised and will inevitably lose rental until it is re-let. It seems to me that the assured six monthly tenancy agreement(a legal document) is most protective of the tenant but completely useless in providing any protection for the landlord. My question as a landlord is - why should I bother with another 'legal' tenancy agreemment. Has any other landlord experienced this and how can I as a landlord prevent this from happening again?