Sec 21 notice

Possession - Section 8 and Section 21

I have a tenant who has two children. The house is a 2 bedroom but the smallest bedroom is a single really. I have spent a lot of money recently in this place athough it was livable by far had I not.
The tenant was and does recognisee the property is small for her now. Its made worse as her boyfriend stays at the property but he is not on the tenancy as she does not want him on it but he is there 24/7. According to her he isn't. He arrived on the scene approx 2 years ago. The tenant is on benefits through Oxford DHSS
I served sec 21 on a tenant which ends on 11 August 21. The tenant a several months ago excitedly called me and followed it with a text saying her boyfriend is buying a house and she would now like to give me 1 months notice she just wanted to sort things out with her universal credit.
She said she would be out by 11 August then she asked could she stay until 11 September. I agreed to this obviously as it saves me chasing a court order and better she moves ammicably. I said there is not rush as I thought she is obviously going so I said let me know 1 month before you are going. She emailed some confirmations on this. She has has always paid rent on time. She has always been good until the boyfriend came on the scene and he I'm sure is behind her actions now. When he arrived she began reporting jobs . All these jobs were addressed. Yesterday for 1st time no rent paid. Where do I stand ref sec 21 now? Do I need to start again or do I need to wait see if rent is paid soon and run with her plan. She says boy friend has lost job and starts new job in August. He is core of problems.