problem tenant

Possession - Section 8 and Section 21

* advice needed * my tenant has been letting my property since FEB 2012 and is now on a periodic tenancy following the expiry of the AST. The lions share of the rent i receive direct from LA and the top up is met by her mother direct to my bank account. Stupidly I didn't process her deposit ( no excuse but I was renovating my home like a madman ready for my newborn due March 2012)
The relationship was always good until SEP 2014 when i have had little contact since - ignored texts, voicemails and letters. It transpires she no longer has 2 children living with her as she is alcoholic and has been "rescued" from the property on a few occasions by the police/ambulance service with little consideration for my front & rear doors. I want her out asap, she won't allow access to review condition nor to complete a gas safety cert. I no longer deal with her but her mother who has emailed to confirm that I should keep the deposit to put right any damage ( new door etc) when she gave 1 months notice. This notice was later withdrawn following the tenants untimely return to the property from rehab. Can I issue a section 21 if I am happy to take any fine given by the courts for Deposit breach ? … if not how can I recover my property ? …. any advice welcome.