As claymore says in #8, sometimes judges get it wrong when the tenancy periods do not coincide with the rental periods. I am giving you to looong explanation as you may need to argue this in court. That's one of the invited complications.
Under s.21(4)(a) Housing Act 1988, the notice given must give the T at least two [calendar] months and be expressed to expire "after" the last day of a period of the tenancy.
(4) Without prejudice to any such right as is referred to in subsection (1) above, a court shall make an order for possession of a dwelling-house let on an assured shorthold tenancy which is a periodic tenancy if the court is satisfied—
(a) that the landlord or, in the case of joint landlords, at least one of them has given to the tenant a notice in writing stating that, after a date specified in the notice, being the last day of a period of the tenancy and not earlier than two months after the date the notice was given, possession of the dwelling-house is required by virtue of this section;...
So you need to work out the dates of the tenancy periods. Section 5(3) Housing Act 1988, defines the terms of a statutory periodic tenancy. See s.5(3)(a) and s.5(3)(d):
(3) The periodic tenancy referred to in subsection (2) above is one—
(a) taking effect in possession immediately on the coming to an end of the fixed term tenancy;
(b) deemed to have been granted by the person who was the landlord under the fixed term tenancy immediately before it came to an end to the person who was then the tenant under that tenancy;
(c) under which the premises which are let are the same dwelling-house as was let under the fixed term tenancy;
(d) under which the periods of the tenancy are the same as those for which rent was last payable under the fixed term tenancy; and
(e)under which, subject to the following provisions of this Part of this Act, the other terms are the same as those of the fixed term tenancy immediately before it came to an end, except that any term which makes provision for determination by the landlord or the tenant shall not have effect while the tenancy remains an assured tenancy.
Re s.5(3)(a) - In other words, the periodic tenancy starts immediately upon expiry of the fixed term tenancy; note, it's a new tenancy which replaces the fixed term tenancy.
Re s.5(3)(d) - ...and the periods of the tenancy are the same length as the rental periods; in this case, four weeks long.
So, the first period of the periodic tenancy was Monday 15th August - Sunday 11th September 2011. You now need to work out the dates of the subsequent four-week periods.
(Incidentally, the rental periods carry on as before).
If MSaxp's dates are correct...
...and you served a notice by hand tomorrow, 25th Jan, you'd calculate it as follows: two months from 25th Jan takes you to 25th March, and that's not the last day of a tenancy period, so you go to the one after that, Sunday 21st April 2013. The notice expiry date would therefore be "after 21st April 2013".21/04/2013 Sun