Possession Claim Struck Out on Invalid Section 21

Possession - Section 8 and Section 21

I have a tenant who moved in on the 1st November 2007 on a six months AST. He is now on a periodic tenancy. I served a section 21 notice dated 28th August 2008 and the notice reads:

"You are hereby given notice that the undersigned is terminating your tenancy of the premises located at Room E, 47 Byron Road, Wembley, Middlesex, HA0 3PB, where you have been a tenant since 1st November 2007. This termination is to be effective after the 1st of November 2008. You must continue to pay rent until this date. The tenant moved in on November 1 2007 and it stated on the notice that it expires........."

The court has struck out the claim stating:

"Invalid notice under seection 21 housing act 1998. This is a statutory periodic tenancy and section 21(4) applies. The date specified must be the last day of teh period of the tenancy."

Can anyone tell me what I have done wrong?