Non Payment of Rent

Possession - Section 8 and Section 21

We started a AST with my wife's friend in May 2008.
She had separated from her husband and has 3 children. The decision to provide her accommodation was an emotional, not financial one therefore the AST was not witnessed, we assisted to move her into our house, no inventory was made of the home etc - this was not a landlord - tenant relationship! Yes, we know now what a big mistake this was!
We reduced the rent to allow it to fall within the housing benefit for the area.
We allowed the AST to expire and for the past two years she has paid regularly into our account as she received housing benefit directly.
We have overlooked the mess, the dirty laundry / cutlery and the overall unhygienic way she kept home; my wife passed comment which led to tension between the friends. But overall she has lived in the house in peace.
Now, the tenant has missed a payment which was also a mistake on our part; rent was being received four weekly and a payment was missed in November 2010. Also, she has not paid since the last week in March 2011, payment was due for April and now for this month also.
We have served her with s.21 which expires in June however what do we do to claim back the unpaid rent?
Apart from writing to her, what else can we do?