Can I ask for six month's notice from my tenant?

If a tenant does not wish to remain in the property after the end of the fixed term, they should notify the landlord as soon as possible. Tenant's should give at least one month's notice to quit, but can provide their notice sooner if they wish. 

Landlords and tenants can discuss tenancy renewals in the Message Centre, but a request for six months' notice is not legally enforceable.