tenant in prison

Possession - Section 8 and Section 21

I have a tenant with an expired AST back in january 13. I already served section 21 notices at the begining of the contact. He is in prison since january and will not be relased until july. There is no one living in the property now.
My Question;
Does this fall under "abandond the property" and i can take it back after 28 days (if there is such a thing)?
If not than;
Do i have to issue the court form N5b to the prison address or to the property address as the "service address" (bottom first page)?