Is there a proper way of reminding tenant that moving out date has passed

We served section 21 before 31 Dec 2010 because tenant was abusive to other tennants in the house. The tenant was supposed to have moved out of her room in the shared house on 28 February 11. She has not paid the rent for February yet and we dont think she has any intention of doing so.

We have had no answers to our texts and messages about a suitable time for the checkout. The tenant said at beginning of January that we could come do viewings any time as we have keys. We have not done any viewings using our keys as we dont want to do so when she is not there.

Is there a nice way of writing to her reminding her that the date for moving out has now passed and that we will be starting posesssion procedue through the courts. This could incur her costs and if she does get in arrears will affect her creditv rating. Have you got a template? I know with s8 there are several prescribed steps. Is there something similar for s21?

If there is no sequence of letters for s21, then is it ok for us to write a letter saying the rent is late etc. We dont want to be bullying but some tenants get bad advise from their friends and then ignore the s21 so we just want to tell them what the next step is and what the implications of it could be. Our eviction agent says just to start preparing court papers but he would say so because then he earns money from it.