When can I serve a Section 21 in these circumstances?

I wonder if I could get some advice as not having been in this predicament before - I’ve rented my property for 11 years and to the existing tenant for nearly 2 years. I gave her notice to leave at the end of March on the 5th October 2021, thereby giving just over 6 months notice. Notice was given through the messaging on Openrent directly to the tenant and her guarantor. I need her out because of rent arrears; she always runs 2/3 months behind and I’ve had enough of it now with a mortgage on the property.
She’s now told me she can’t leave as nowhere to go and the Council have told her to stay put.
When can I serve a section 21? She was due to leave next month so can I serve it now with 2 months notice, taking her leave date to November and in between rent payments due. Rent payments /tenancy started on the 6th of the month. Do I have to wait until 6th October before I can serve her a section 21? -Therefore it means she has till December now to stay in the property.
Is the 6 month notice I gave her valid through Openrent or should I have put it in writing?

Thanks for any help, anyone can give!!

Rant now on ! typical of the council. leave the landlord stitched up. Do nowt till they have to. Push the problem into the future . I will never help any local council with a place for those who need it. That is my only weapon.

You must serve a proper notice and be COVID aware ( get a solicitor if you are not sure)
Form 6a for s21 and form 3 for s8

Serve s21 and s8 notice on October 1st. The notice period goes back to two months at that point
If there is greater than 4 month arrears you can serve s8 now

And serve it on paper either with an independent witness or a receipt of 1st class postage from a post office, or its likely invalid