Business tenants have the protection of the Landlord & Tenant Act 1954. To get a tenant out the landlord has to give formal notice, but the tenant is entitled to serve a colunter notice requiring that a new tenancey is granted to him, and then to apply to the court for such a tenancy. The landlord can not object merely because he wants to sell the site, but there are a limited number of grounds that can be used. The tenant can also start the ball rolling by serving a notice requiring the landlord to grant a new tenancy, I suggest that you get good legal advice now