if you dont know the law regarding tenancies (i.e. what to do if the tenant doesnt pay the rent, damages your property, wont leave at the end of a tenancy etc etc) then you may be better off paying a reputable agent. Look for one that is a member of a recognised trade organisation such as ARLA.
If you do know the law, then you'll save money by not using an agent.
You can do credit checks yourself through reference agencies. These start at about �15 with the more comprehensive report costing about �30.
If you use an agent then, initially at least, they should chase the tenant for rent. However, if it goes past this stage and you have to go to Court, then this is down to you as the agreement is always between landlord and tenant.
There are plenty of pitfalls with letting if you dont know what youre doing.