By no means legally binding and again not clearly defined, but this is what they have included in the Firearm Safety Booklet 2011:
"Sights must be correctly set. If they are not, rounds may fall short or go far beyond the target. Adjust the sights for distances suitable for the calibre and intended use.
‘Sighting in’ is best carried out on an established range. Otherwise, it should be somewhere away from people, stock or property, and with a safe backstop."