well if i were you i would stick to targets but keep them low. i dont shoot anything metal apart from cans. but anyway just get a decent backstop like a bit of wood coverd in carpet or a compost heap (anything to stop the pellet)
another thing i want to ask is why were you shooting anywhere near the direction a pellet could go to hit someone on a ladder? you must have been shooting towards them or in the diirection of them for it to be able to hit them.
Read the BASC website.
http://www.basc.org.uk/en/codes-of-practice/air-rifles.cfm
it tells you all the laws and recomendations.