I've had a rental property for the last 5 years or so.
My advice would be to do everything yourself, from organising putting the deposit into a deposit protection scheme, the property inventory, the contract, the management etc.
The only thing I use agencies for is actually finding the tenants. They have the benefit of being able to advertise on Rightmove and they have the time to show people round. However I avoid them for everything else. Obviously you could find someone yourself too but I personally don't enjoy showing people around houses so I leave that part to the agent.
They will try and talk you into some kind of management scheme for 10-15% of the rental fee but in reality they'll charge you through the nose for something which you can quite easily do yourself.
The truth of the matter is, if the property is in a decent state then the management fee is a huge rip off because there is no actual management taking place 90% of the time. And even if they do take calls from your tenants now and again, they can't make any decisions without speaking to you anyway so they just become a middle man and charge you a fee for the privilege.
There's loads of information on there ranging from general landlord advice (specific to the UK) to full document templates which you can use (AST, Inventory, Reference forms etc). For example an agent will try and charge you a small fortune to organise the contract (the first time I rented my place out I was charged £200 just for by the agent just for the contract), when you can get an equivalent from the RLA for free if you're a member or £9.95 if not.
Get to know your tenants a little too. If they think you're a nice friendly bloke then they're more likely to look after the place in my experience.