We've done this a couple of times on the internet - someone has already posted the link. One time I knew the address, but on other occasions we have give the rough address and sent a map with the area highlighted. They will reply with the OS field number, and name and address of owner. You can then just pop them a letter.
Found it quite easy, and it's only something like £3. No bugger round here wants to sell their land though!
Good luck,
Sarah.