Section 21 or not

I have a Tenant on periodical she has been a Tenant for a couple of years. About 14 months ago she had a problem paying her rent being a nice Lanbdlord i agreeed she could miss a month then pay £50 a month extra each month until it was paid off. It started ok then petered off i did not do much just wrote to her and went and saw her regulaly but i was not that worried as she had a well paid civil service job. She then lost her job and went on houseing benifit. She makes sure that she is always less than 2 months behind. Can i evict her for rent arrears.