LHA HB & Partial Rent Arrears

Rent Arrears

Our tenant moved in in June 2012, with an AST, and deposit paid by the LHA. She has 2/3 of her rent paid by the LHA and is responsible for paying £120 every 4 weeks. She was honest about her history (not good) but we decided that we were prepared to give her a chance. However, she has been getting late paying, only made a partial payment in February and is now 2 weeks late with her March payment, owing a total of £170. Is there any particular wording to be used when issuing a written warning, that will help in any subsequent proceedings, whilst also encouraging her to be more reliable. If we have to wait for two month's rent arrears is this just her part of the rent (2 x £120) or twice the full rent (£840) which would mean that she would actually be seven month's in arrears?
Many thanks, The Sailor
PS The lesson is don't mix charity with business unless you're prepared to make a loss on your business!