Issuing of section 21 notices

Possession - Section 8 and Section 21

I have several questions relating to section 21 notices which I would appreciate your help with, these are:

1. Could you clarify when a section 21 notice expires for a periodic tenancy. if the rent payment day is the 5th of every month, should the expiry date for the section 21 be 2 months in the future on the 5th day of the month i.e. todays date 30/07/09; next rent payment date 05/08/09, section 21 expiry based on these dates = 05/10/09?

2.Can a managing agent issue a section 21 on behalf of the landlord? Does special permission need to be sought for a managing agent to do this?

3. Does section 21 notice only apply to AST?

Many thanks for your assistance.

Kerry