possession of flat

Possession - Section 8 and Section 21

we have had an offer to purchase a flat which is currently let to a single mother and two children, the tenancy staretd on 26/9/08 for 6 months, we would like to give notice so we can get vacant possesision as the new buyer would like to move in by 1st december.

what forms do i use and how much notice is required?

do teh forms have to be sent by registered post?

She is a very good tenant and hopefully there will not be any problems, but we did forget to register teh deposit, altough i shall repay this in cash, do you forsee any problems with this?

many thanks

geoff