Someone will own the land. If it's registered, you can find out who that is from the Land Registry:
http://www.landregist...erty-or-piece-of-land
If it's not registered your local County Record Office might have maps which provide some clues as to who the owner is.
If you fence the land and use it for 10 years (if it's registered) or 12 years (for unregistered land), without challenge, you
might be able to claim title to the land by 'adverse possession'.
Start here:
http://www.landregist...uides/public-guide-16
If you need the 'small print', follow the link from Section 8.
Chris
PS: Since trespass is not a criminal offence, you'll be doing nothing wrong if you make use of the land (irrespective of whether you later make a claim for adverse possession). However you risk wasting money on fencing (or, for example, by having to dig your crops up) if the owner then challenges your use of the land.