section 21b

Tenancy Deposits

I have served a section 21b notice on a tenant by posting through her door. This was witnessed by a friend. The notice was posted through the door on the 18th August 08. Rental payments are due on the 18th of each month. The tenancy agreement had expired and had reverted to a month by month tenancy. The tenant has been in touch with the local authority housing office. They have advised that the section 21 is valid but have also told her not to leave the property until I obtain a court order for possession. The tenant will owe £1285 in rent arrears by the 18th October. The housing office has arranged an emergency payment of £60 to be made next week!!
What do I do from here?
What proof would I need to give to the court to obtain a possession order?
Thanks