Arrangement for repayment of arrears

Rent Arrears

My tenant has been a model tenant for two and a half years , but then her financial support from ex husband was withdrawn and for some reason she failed to get any benefits or housing benefit . She has now gained employment allegedly permanent starting at end of November.
The arrears are now 4 months and will take along time I suspect to pay off .
I intend to make a new lease agreement with a higher rent to incorporate arrears repayments and to make them legally binding -- but do you have any further advice on how long should the lease be for , and should I also set up a section 21 notice at the same time as you always advise ? Thank you .