You don’t need an agent to let your property to tenants, but it can mean plenty of potential headaches are minimised or even eliminated.  
You'll be charged between 10% and 15% of what you get in rent each month by the agent so don't forget to factor this into your calculations. 
Most agents offer two types of services: 
Basic service This is a tenant-finding only service. Once you have your tenant, you'll need to manage the property and tenancy and you'll be required to liaise directly with your tenants. 
Full management This includes a full inventory of contents, regular inspections, arranging maintenance inside and out, as well as sorting payments and handling any complaints. It usually costs between 6%-15% of your annual rent. 
Professional agents use a contract. As a landlord you should always read the contract carefully and make sure you're not agreeing to any unreasonable terms or charges or taking on all of the responsibilities.
You will have to declare what you get in rent to the taxman but you can deduct expenses. 
Keep a record of everything you spend which includes letting and management fees as well as the cost of any repairs.
As tax is a complicated issue, it might be best to seek professional advice.
For information on the tax implications of property rental take a look at www.directgov.co.uk

Source:- http://www.which.co.uk/consumer-rights/problem/i-cant-sell-my-house-can-i-rent-it-out