If the tenancy started before March 1997 it is likely to be an 'assured' tenancy. (A tenant who had been there as long as this could only have an 'assured shorthold' tenancy, giving them fewer rights, if you had informed them before the tenancy began, using a special form called a 'section 20 notice').
An assured tenant has more rights than an 'assured shorthold' tenant - as long as they keep to the tenancy agreement they can usually live there as long as they want. If they die, the tenancy can pass to family members who lived with them. If you want an assured tenant to leave, you must have a good reason, and get a court's agreement to evict them. Find out more about ending a tenancy on Gov.uk.