cheating agents

Possession - Section 8 and Section 21

i have a property in derby, the agents have told me nothing but lies since day one, the lies are getting more outrageous, the tenant is now 5 weeks behind the agent is holding £320 which they are refusing to give me, as "it affects their accounting process"
i want to evict the tenant, the solicitor is suggesting either section 8 or 21, the problem with the section 8 is the tenat may pay whereas the section 21 will i think be more effective
would you suggest 21 r 8 and if this dodgy agent has put someone else in will the paperwork still apply
i have signed no paperwork with the agent and they have held me to ransom , i have sent a solicitors letter to the teant and had no reply
what would i do to cut the agent out and also on their tenancy agreement there is no deposit

your expert knowledge please