Double negative

Yesterday, as I got home from the supermarket (I know! Such an exciting life I lead!) two yoofs were sat on my neighbour's front wall. Since she's had trouble with racist graffiti, rubbish being left on her lawn etc. I kept a sneaky eye on them (while trying not to look like some deeply paranoid middle-aged dullard - look, I work with young people, so I'm not pointing the finger, just trying to be neighbourly).

One one trip back to the car boot I could clearly, definitely, smell weed. Hey, I've been to university people.

'Seriously?' I said to the yoofs, bearing in mind it was broad daylight, middle of the day, not exactly being subtle.
Them: 'What?
Me: 'I can smell it.'
Them: 'What? Smell what? We ain't done nothing!'
Me: [biting down the urge to pedantically state 'I think you'll find that's a double negative, meaning you actually have done something'] and yes I know language changes and adapts, and if it didn't we'd all still be speaking like Chaucer and I should just get over it, but I'm still a copy-editor and I don't like it, ok? 
'Er, the dope?'
Them: 'Nah, man, no way'.

As I dredged nappies, kitchen roll and frozen food inside, they quickly hoofed it round the corner. I wouldn't have called our PCSOs (who were utterly lovely when some other yoofs told me to fuck off in front of my kids and threatened me) anyway, but I did chuckle to myself a bit at the idea of a) them being so bold/brazen/stupid as to smoke it openly in the first place and b) them legging it so quickly when they were 'rumbled'. 

Roll on term time...