My advice on setting up a new field...
Make sure you give out detailed and clear directions, wherever you end up getting land. Nothing is quite so frustrating as driving from home on a chilly morning then searching forever to find some outfit out in the woods that's hidden like a CIA training camp.
Consider leasing from a farmer... he's not gonna "steal your idea". He has a business to run, and you renting a corner of the field that's all full of trees and creeks and old cars is gonig to increase his income without him lifting a finger. Just make sure you can show him insurance documentation. He will want to cover his own liability.
Get over the notion that somebody is gonna steal your idea. It will not get them anywhere if they can't steal your work, too. A fair amount of that will be required before you get open.
Get markers that are easy to care for... I recommend Tippmanns, but that's personal preference. Infernoes, Pirhanas, or whatever, be sure you and the marshalls can tear 'em down and clean 'em up without staying at it all night. If your rentals don't work well enough, no one will bring you new business. If they work too well, no one will order sexy angels and cockers and timmies form the pro shop you open next to the field in a few years.
There is a book from Prostar about opening a field in the States, and I can only imagine that most of the advice therein will apply to the UK, too. Guy Cooper wrote it, and it gets updated from time to time.
http://www.paintballadv.com/ should take you to Guy's proshop in Denver, Colorado, USA.
As far as competition from other operators: it sounds like you are far enough away that you are building your own market, rather than diluting someone elses customer base. That is good and bad. Remember that each person introduced to paintball by you is going to shop all over, and vice-versa. Paintball is new enough that almost all growth of the industry benefits almost every member of the industry. Don't worry if another field opens nearby. It will build your customer base in the long run. If you think big, you will make it through the long haul.
good luck! I'd come check out your field, but you know... Atlantic Ocean and all.