Rent Increase After s21 and Issue of Proceedings

Hi,

Can I seek or accept an increase in rent after having previously served an s21 and issuing eviction proceedings (I got an order for possession in August) but have not yet applied for a bailiff)? The rent was increased in September – does receiving a higher rent invalidate this or create a new tenancy?

I agreed and increased this verbally/via text as the tenant said she wanted to be rehoused by the council, or must it have been in writing?

The tenant has paid it last two months, but now (3rd month due) is saying she is not obliged to pay as it wasn’t in writing.

I can send her an s13 notice of rent increase but do not want to invalidate the court order (I hope it is still valid despite the above – she probably won't contest it) as she is difficult at times and may need to send bailiff shortly anyhow even though she's paying nearly £150 a month less than market rent, so I wanted to see if she would get any better. Still, she hasn't and now has created another issue.

Thank you.