While it is too much use of the horn, what is the accepted way to tell someone they've forgotten their lights are off? Flashing them (wahey) can be a bit like staring into the sun so I wonder if it's really the best way (although not sure the horn would work either).
I tend to switch to sidelights quickly rather than flash.
It's more obvious at night (a flash could just be taken as you going over a bump in the road), I feel it gives a clearer indication of what the problem actually is (hey look, no lights!), and you aren't going to dazzle the other driver.
Who's taking bets on whether OP is driving a BMW or a White Van?
Yeah, got to be something German, or a "hot" hatch (which is actually just the base spec model with the smallest engine after being driven through Halfords)