Section21

Possession - Section 8 and Section 21

My tenant is 3 months in arrears. She is awaiting HB. This has been confirmed by the council but she hasn't responded to further info requests from them. I want to serve her with a Section 21 shortly as her AST tenancy runs from the 11th of the month.
She has told me that she will go to council today and sort out what's required. I intend to serve the s21 on the 10th of this month. If the HB is paid I would be happy to let her remain in the property. Would this be ok if she has had a s21 served. if it is ok does she just remain on the original AST agreement.

thanks