Worked in this area, oink.
Right. Regardless of what people like worcester say, if your landlord has not served you with a Section 21 notice then you have not been given a valid notice. These laws are there for a reason.
The Section 21 can be given at any time during the life of the tenancy, but it must give AT LEAST 2 months clear notice. The notice must be dated very, very specifically to expire on the last day of a rental period & say that possession is required AFTER that date.
So, the only legally valid notice he could give you would say that possession is required AFTER 25th of whatever month at least two months hence.
There is still time for him to give you legal notice for the end of March. If he sent it today it would need to say that possession will be sought AFTER 25th March 2013.
I'm sorry, I have to go to work now but this clarifies things further:
www.letlink.co.uk/letting-factsheets/factsheets/factsheet-21-section-21-notice-requiring-possession-of-an-assured-shorthold-tenancy.html
Your tenancy is currently an assured shorthold one. It will automatically (without you or LL needing to do anything) a periodic one after 25th Feb.
Also, for clarification, and because it is very, very important (lots of Section 21s are slung out of court for having the wrong date on them) a rental period runs from the day your rent is due to the day before the next one is. For you this is 26th of the month to 25th of the next.