Serving a Section 21 Notice

Possession - Section 8 and Section 21

Hi
I have just joined the Assoc.

I need to serve a Section 21 on my tenants, but am unsure whether to use Section 21(4)(A) or Section 21(1)(B)

It is a Furnished Assured Shorthold Agreement which expires at the end of May.

On the form where it says "I/We require possession of the dwelling-house known as........................After......
do I enter the date the form was handed to the tenant or if posted (advised to send 2 copies from 2 different P'O's) 3 days after posting or the date I want to take possession of the property?

One other question.

The tenants have been at the property for 4 years, one has since moved out but his name is still on the original agreement. No new agreement was issued. Would this cause me any problems?

Many thanks.