Timing of S21 notice.

I have a tenant who has been in arrears with the rent for several months. The tenancy is for 12 months, expiring on 23 November 2008, with rent payable on 23 of each month.

I served a S21 Notice on 13 October, to expire on 13 December 2008.

The tenant has gone to the District Council to get rehoused, but they have told her that the S21 notice is not valid because they say it must coincide with the date of payment of rent. They are suggesting I should issue a new notice dated 23 Novmber to expire on 23 January. As a result they say she is not homeless and they will not rehouse her.

I have looked at RLA guidance and the Housing Act and cannot see any requirement to issue the S21 Notice to coincide with rent payment dates.

Please could you clarify the situation and provide me with relevant legislation details to support my case and to argue with the District Council.

Thanks