rent arrears

Tenancy Deposits

Tenants moved in 20 april 2009 there was no deposit and no money was paid for the first 8 weeks, the rent was £475 per calender month I applied to the benefits agency who at approximately 10 weeks paid £172 and then stated they would pay £400 on 20 july which they did and would continue to do every month the tenant agreed to make the rest and pay £190 on 28 july towards this.
I received a letter from the benefits agency today stating they stopping payments forth with and i have not received the £190 as promised. I need to start proceedings to get these tenants out but they have not been in the house 6 months so i don't think sect.21 applies. Can i use sect 8 to get the process moving.
Clare Hunter