Claiming possession when tenancy start unknown

Tenancy Types and Management

We have a car showroom with offices above that we have had rented out on a long term lease that has now expired. However, the tenant has been subletting the offices above for many years as a flat and we now wish to evict this tenant. We were aware of this subletting but had turned a blind eye and we have no idea when the tenancy started as there does not appear to have ever been any written lease. We would guess that the tenancy started after 24 February 1998 but not 100% certain. What is the best course of action to claim possession? If it is to serve a Section 21 Notice, what date should the notice expire?