Section 21 notice

Tenancy Types and Management

I have a tenant on DSS with 2 children living in Manchester. The Housing Benefit have not paid me since 18 August, because my agent changed his address, and they informed my tenant that the address was not correct. My agent has sent two letters to HB copied to my tenant of his correct address. As usual HB has not answered the letters. We therefore sent letter to tenant giving her 7 days to go to the HB and find out why the rent is not forthcoming. She has not replied and the phone is switched off. As she may be playing some game, possibly go get a council house, I want to evict her with the least possible delay. She also owes rent from beginning of tenancy. As there is no chance of her paying me these arrears I want to just issue a Section 21. Her contract started on 14 October 2004,for 6 months which has elapsed into a periodic tenancy. Where does the two months start, and which is the expiry? As I suppose I will have to apply to the court, do I have to attend a hearing or is it accelerated possession?
Thanks for your kind help.