As well as facilities I'd say safety and insurance are, going to be your primary concern. Make sure that wherever you hold this you have at the very least a first aider there for any sort of injury that may occur and the means to get someone to hospital if necessary; one larp I was in a friend fell and shattered their shoulder while just running down a hill, and he was a veteran larper.
Insurance is a consideration if you're playing with anyone other than a bunch of friends, a decent short disclaimer form (You may suffer injuries, organisation not responsible) to get you out of any unlikely but sadly possibly quandries if someone you've never met before turns up and causes a fuss.
Also; I forgot one other important thing; I see you're from the UK (Horah for another Brit, welcome!) so if you're doing any kind of scifi and waving around pretend guns make sure that you're either well out of sight of the public, have signposts or have made the police aware of what you're doing - one of my friends was running a cyberpunk style game and had the armed response unit turn up, that was not fun!
Also: Inspection! People will bring their own gear, it's wise when people you don't know are attending that the stuff they are bringing is safe. The usual way to verify this is to hit them with their own gear ;) But I've heard horror stories of gaffa taped steel cored weapons and the like - safety again!
First aid! For scratches, cuts, allergies and bites.
Toilets! You have to go sooner or later.
Fresh water. Especially if you're playing somewhere hot.
Shelter. For when the water starts falling from the sky.