Sorry rockolo, that is too simple.
Of course you can use a knife or a poker, if that is reasonable. The Police for example, do not really have any extra powers than a civilian in these circumstances, and they carry and use bloody guns!!!
Again if the intruder is walking away you can still use reasonable force in their apprehension (arrest if Police).
Sect 3 of the Criminal Law Act (amendment)
http://www.publications.parliament.uk/pa/cm200 506/cmbills/018/06018.i-i.html
Has always mentioned the use of force re arrest. That means if the "attack" kicks off again when you are trying to arrest him, we are back to square one and game on.
The case of Tony Martin as you mention was, in the eyes of the law, unreasonable. If he used the gun in their apprehension and then the pikey sh!t was shot, no offence.