Should I use an agent or do it myself?
Like everything, there are pros and cons with regard to using a letting agent. It's very important to weighthem all up carefully.
is the rental property local to you?
If you don't live locally to your rental property it is almost essential to use an agent, to avoid hours spent travelling to show the property to prospective tenant - a good reason for investing locally!

Agents take most of the hard work out of the letting process. Also they can be there to 'hold your hand' when two weeks have passed and no one has wanted your flat!
Dodgy tenantsDodgy tenants are less likely to go via an agent mainly because of their stringent referencing procedures. And of course agents are more likely to be aware of 'well-known' rogue tenants.
These days, particularly since the regulations that came into force since 2015, there is a LOT of 'red tape' involved in letting a property and letting agents can usually organise all this for you.
Ongoing managementThere is a section of tenants, particularly those looking for more high end property who, despite the costs, seem to prefer to go through an agent. This is probably because they like to keep a distance between themselves and the landlord. Others may be concerned that the ongoing management of the property may be more reliable if they are going through an agent. This is sometimes true, sometimes not.
The cons of using an agent
Number one - agents are VERY expensive! They will charge you, the landlord between 7% and 10% for a let only option. Let's average this. Let's say you agree 8.5%, what does that actually mean in terms of hard cash? Say the rent is £1000 pcm. You will be charged £1020 plus VAT, making £1224 for the first year of tenancy. You may well be charged this although probably less each time that tenant simply renews!