Date to put on section 21 (4) (a) notice

Possession - Section 8 and Section 21

thankyou for your assistance.
a 12 month fixed tenancy expired on 23rd feb 2012. the rent is due on 25th day of each month. we tried on 3 separate ocassions to invite the tenant to clear rent arrears.
we have a section 21 notice served in 2009 but this says nothing about tenancies next occurring in the future, so we think we should reissue a fresh notice so there is no ambiguity. we do not want to sign a new 12 month tenancy and wish to serve notice for possession. if we hand deliver and get their signature today on the notice, will we confirm the date as 24th may 2012? if the rent is next due on the 25th of each month, which is obviously much longer than 2 months, or can we have the date as 3rd May to mean 2 months and a day from today?
your assistance will be very welcome as we don't serve these very often, i'm glad to say, after a fixed tenancy has expired.
regards,