Are you going to be leaving the country? If you can "drop by" the property every once in a while (by agreement with the tenant(s)) then it helps to ensure that it's being looked after. If you're not around, however, then an agent is very much the better option. We let a former granny annexe out for just under 4 years. Initially we used an agent but latterly did it ourselves. When we had the agent I begrudged every penny I spent on him, judging it money for old rope. When I didn't have an agent I reminisced about the "good old days" when we could just fire off a letter to the agent and spare ourselves any awkward interviews with the tenant! Human nature, I suppose.
Other things to consider are
1 mortgage - do you have one? - if so,you need to tell the lenders that the property is let - they may very well increase your mortgage rate as a result - usually only by 1/4% or 1/2% but it needs to be factored into the cost effectiveness of the letting.
2 House insurance - again you need to get in touch with the insurers - your current policy may not be suitable and you may need to take out a new or a supplementary policy.
3 furnishings - if you are letting the property furnished then fabrics, materials, beds etc need to be fire certificated - not all older materials are.
4 appliances - you will need current testing certificates for things like gas fires
5 You should have smoke alarms fitted for your own safety in any case, but if you are letting you have no choice.
Points like 3,4 and 5 are areas which make involving an agent at least initially a good idea.
Final point - if an agent finds you a good tenant and you want to continue letting to this person but "cut loose" the agent - you may find that the agent will require an additional fee on termination of the contract.
Good luck!