In reply to JJL:
I used www.openrent.co.uk.
They are not agents but they will advertise (via rightmove and others) for you and you can get them to organise contracts, do gas safety checks things like that. They used to do the advertising for free the first time you put your place up.
I found them really useful, everything is done online and is really convenient and you just pay for the stuff you want.
It really minimised the hassle of everything for me.
Make sure you get a good tenant, that's the most important thing!