Rent arrears and section 21

Possession - Section 8 and Section 21

Hello,

I have a tenant who is now in arrears with her rent and I wish to regain possession of my property. I have read all the advice given on this site and it appears that s21 is the best route having issued this paperwork at the beginning of the AST. My tenant is now on a rolling periodic and this took effect before the striker case. Having read the advice on this site I realised that I should have re issued the PI. Having seen Dave A's advice to another member I reissued the information this morning to her address having a witness with me as I posted it through the door. The question I am asking is, can I now issue the s21 legally, should I wish to go down this route, with a view to regaining my property back.