Dodgy letting agent - want to get rid of them and tenant

First time on here! I rent out a flat using a management agent. For the last year I have had to chase my rent from them every month and when I contacted the tenant, turns out she has paid him upfront so he has the money but is witholding it saying he has bank problems every month.

Anyway, I want rid of both the agent and the tenant (who is OK but have a feeling will struggle to get money back from agent and afford the rent every month).

The contract with the agent states 2 month notice and the tenancy agreement ended on 11th December 2012 so I think if I serve an S21 4A periodic notice to the tenant tomorrow requiring possession on 12th March 2013 I am abiding by the rules? And if I just serve a normal notice letter to the agent, will that do? Can anyone advise?

I am worried also about not getting my rent from either of them - how can I enforce that? And as long as I give the tenant 24 hours notice, I should be able to get the new agent in to get viewings - is that right?

Grateful for any advice!

Thanks