Before you go metal-detecting
"Adhering to the laws concerning protected sites (e.g. those defined as Scheduled Monuments or Sites of Special Scientific Interest: you can obtain details of these from the landowner/occupier, Finds Liaison Officer, Historic Environment Record ...)"
While you are metal-detecting
"Wherever possible working on ground that has already been disturbed (such as ploughed land or that which has formerly been ploughed), and only within the depth of ploughing. If detecting takes place on undisturbed pasture, be careful to ensure that no damage is done to the archaeological value of the land, including earthworks."
"Minimising any ground disturbance through the use of suitable tools and by reinstating any excavated material as neatly as possible. Endeavour not to damage stratified archaeological deposits."
After you have been metal-detecting
"Seeking expert help if you discover something large below the ploughsoil, or a concentration of finds or unusual material, or wreck remains, and ensuring that the landowner/occupier's permission is obtained to do so. Your local Finds Liaison Officer may be able to help or will be able to advise of an appropriate person. Reporting the find does not change your rights of discovery, but will result in far more archaeological evidence being discovered."The Lenborough Hoard discovered just before Christmas appears to have been on a site that includes a deserted medieval village and "ridge and furrow" field system. (The Historic Environment Record for Buckinghmashire notes, "Medieval house platforms, hollow-ways and ridge and furrow seen on aerial photographs".) The fact that the hoard was found some 60 cm below the (undisturbed) surface appears to be significant.