Section 21 clarification & reasurance

Possession - Section 8 and Section 21

The RLA shorthold tenancy agreement was completed and signed on the 11/5/08. Tenancy commenced on that date at 11am and ran to the 10/11/08 when i presume it reverts to a periodic. Rental payments are monthly and for ease they are payable on the 1st of each month.
We want to gain possession of the flat as there has been a major water leak which means the bathroom has had to be gutted to allow walls/floors to dry out. The tenants have verbally indicated they are looking for a larger place but now i want them out to enable me to carry out proper repairs as they have failed to keep the flat clean etc. Can you tell me the dates i need to use as i am number blind and cannot seem to get your explaination in my head. I know you have covered this time & again but numbers are not my forte. I want to do it correctly. I am hoping they will comply as the flat has no shower/bath due to the leak.