advice on severing tenancy

Possession - Section 8 and Section 21

Hi I was due to evict my tenant in August , however this did not happen as the local authority stepped in and now pay his rent , the arrears of £1350 remain unpaid and I am prepared to write this amount off as the tenant has no money

I have correctly collected a deposit of £625 in 2011 and this is lodged with a deposit scheme
The tenancy agreement expires in March 2014 and I would like to tenant to leave and if possible secure the deposit against the arrears above.

what are the legal requirements I need to apply to ensure
1. he does leave
2. I get back the deposit on the tenant
3. is a further eviction notice a better option than serving the section 21 , as I have no change of getting the money back anyway or is this what I should do ?

I may not have explained this very well , so thanks for your help