How much notice do i need to give

Other

Dear Sir / Madam,
I need to give notice to a tenant who is now 2 months overdue on rent payments and would ask if you can clarify how much notice i need to give. The tenancy started on the 5th April 2011 for 6 months and then moved onto a periodic tenancy. A section 21b was issued at the time of the tenancy which the document states expires on the 4th October 2011. The Council had been paying her housing allowance however it was suspended and then i received a letter on Saturday 4th Feb telling me it had been cancelled. The tenant owes rent from the 5th Jan and now the 5th Feb ?.

Hope you can advise asap.

Kind Regards.