when I fought back wi a pick axe handle just prior to getting mi head kicked in by a load of drunks.
Now come on Ste - just cos you know I've a thing for axe-wielding homicidal maniacs!
I suspect the police have been somewhat quoted out of context here to invent a story that doesn't really exist. There's absolutely nothing wrong with grabbing a knife to defend yourself if you're actually being attacked. You can easily justify your actions in Court as self defence.
Picking up a knife and waving it about, while banging on the window and shouting at a bunch of teenagers in your garden however, could be construed as overly aggressive.
You don't know why they're in your garden. They might have broken down, seen lights on at your house and hoping that someone's up and about who might be able to help with jump leads or whatever. Rather than knock on the door, which would be rude and somewhat scary late at night, they're looking in through the window to make sure someone's actually awake.
Whilst it's very creepy to have someone peering in at your windows, you shouldn't automatically assume that they're "intruders" or that you're under attack from them.
Could even have been an attempt at humour to put the lady at her ease, as when you're shaken up by an experience, it helps to lighten the tension with a laugh...
When the police turned up after my burglary, one of them said "Dammit. No blood!"... I was a bit confused (and crying bucketloads at the time!) until he said with a grin "It'd make me feel a whole lot better if the little sod had cut himself on that window he broke" and got me laughing. A lighthearted remark that could very easily be twisted against him.