Need possession of the property ASAP

Tenancy Deposits

I have let a property through an estate agent on full management. The tenant was highly recommended to me by the agent, but I had doubt if the tenant would be able to cover the rent, so the estate agent became the guarantor and we agreed to let the property for 12 months period. When I receive the contract, the tenancy was only for 6 months. My signature was not required and I didn't have chance to see the AST before they put the tenants into my property. The agent said it would go into periodic tenancy after this period.

Every month since I let property the agent makes excuses why the tenant can't pay the rent when due and it is now 7 weeks in arrears. I have been asking the agent I need possession of the property ASAP since January 2009 and they eventually served a notice 2 months ago (tenant is in period tenancy since October 2008). The tenant was due to vacate the property on 7 May 2009, but he hasn't and now the estate agent are not returning my calls or emails.

How can I legally take possession of the house again? can I take over from agent or can I get someone else to act on my behalf?

I would be grateful for any help you can give. Thanks in advance.