Letter with a Section 21 (4)A notice

Possession - Section 8 and Section 21

Hi. My tenant has had difficulty with her housing benefit since her son left school in June 13 and since Aug 13 we have had difficult in getting rent or it has been late.We have been patient and understanding but enough is enough. We are owed £75 from November and all of Decembers rent. We have a Section 21 notice prepared as believe that is the best way forward with the intention of using the small claims court to retrieve the owed rent which will probably be more with rent due on 16 Jan and Feb. Do I need to send an accompanying letter with the Section 21 with vacating instructions or outlining the reasons for the Section 21 or is the notice only adequate at this stage?