Help for repossion

Possession - Section 8 and Section 21

We have tennants that have a periodic tenancy agreement as the original assured shorthold tenancy for 6 months ran out and they continued on without having another one. They havent paid rent for 4 months and we were going to issue section 21 but the courts told us that we couldnt use that method due to a problem with the deposit. We are looking at doing section 8 but we arent sure if we have to send this to the tenants first or what. Please can someone help us and advise us the correct route to take.