trouble to evict long term tenent

hi all,

tenent moved into property 1995
originally had a 6 month tenancy contract with the original owner then.
but hasnt had any additional tenancy contracts since with me or any other new owners.

Have been told by c.a.b his tenant status because he rented the property since 1995 he is either an assured tenent or an assured shorthold tenant.
But because he signed an 6 month tenancy agreement at the beginning he is a assured shorthold tenent or to be exact an assured shorthold periodic tenant because the original tenancy agreement rolls over.
And its quite easy to evict these type of tenants.

But because the origional copy (and his copy) of the original 6 month tenancy contract have been lost, he is claiming to be an assured tenant status where its very hard to evict this type of tenant.
Now my question is what is the best plan of action to evict this type of tenent.
any advice well appreciated.
cheers
gymboy