Housing benefit/proposed rent increase

Other

I have a tenant in a one bedroom flat in London SE7. She has been there since Nov 2010 with no rent increase, paying £650 pcm (paid at £150 pw by mutual agreement). During her tenancy she has lost her job and now receives housing benefit. I am advised by a local letting agent that the rent I could now expect to achieve is £170 to £175 pw. How should I approach an increase in her rental with respect to the housing benefit she receives? Housing benefit is paid to her by the Local Authority.