Constable Norman Gibson: The term that the law uses is 'reasonable force'. Whenever I am teaching shopowners and staff about reasonable force I would explain that the law would look at the circumstances and the level of injuries. There is a need to recognise the difference between lawful force and assault. Whenever a police officer uses reasonable force to restrain someone that is not an assault, it's reasonable force. If it goes beyond the boundries of reasonable force it can become an assault. There is also the question of intent. If someone comes into your house and you lift what is refered to as a weapon of convenience (something close at hand) and they get seriously injured and your intent was simply to protect yourself I believe the law will support you.
However if you announce in advance that the next person that breaks into your house you're going to assault them or seriously injure them, I do not believe the law will support you because you are not looking at the circumstances and your intent is to seriously injure.
found this on
http://www.bbc.co.uk/northernireland/talkni/ask_a_cop.shtml
which kind of explains how the homeowner could have been found guilty.