Have arranged to go walking for a weekend with a friend. No kids, just me and best girl-friend. But where? We want somewhere reasonably accessible from London - preferably by train, and a cheap B&B. Planning to leave London Friday night, walk all day Sat and half of Sun, come back Sun evening. Number one criteria is wide-open space. Lots of it. Any ideas? Brecon Beacons beckon (try saying that after a couple) but where?