From what I understand you can defend yourself/property/family when the intruder is actually ON your property? A lawyer on BBC breakfast the other day said that if an intruder was coming at you with a knife in your house, you could use a knife or even a gun to defend yourself.
The problem with this case is that the intruder had left the guy's property and so in the eyes of the law, the danger had gone and he was partaking in vigilante justice.