IMO in the UK during the day whatever the circumstances it would be a field therefore not en-dangering the lives of those in moving traffic.. .. .However at night especially in the UK with a cloud cover most of the time you would not be able to distinguish a field from a lake until its too late! Not only that but also the black patch (that you assume to be a field) whats in it? Just a field or maybe a house/Barns? or even a small village? If there was a power cut however, you would not be able to see a whole town. So IMO an emergency landing at night would have to be on a lit road even if I do manage to take out a lamp post. If you dont have this option then I guess you have no choice other than to aim and pray.. .. .Take into account also that with a dark field at night with no lighting you will have no depth perception and this will bring you slamming into the ground anyway. Most incidents that happen at night whilst carrying out a normal night VFR landing occur due to the pilot not being able to judge the approach.. .. .The chances of a field being empty is quite high however I'm sure most of you have come across the 50:50:90 rule? 50% chance of getting it right, 50% chance of getting it wrong and 90% chance that you'll choose the wrong one.. .. .Happy Landings to you all!!!