On my way home from someones house there was a Beemer sat at a red light just a few hundred yards from home, I was trundling up in my Favorit and noticed the lights had changed on the other signals.

Went in the the right hand lane so overtook the Beemer.

BMW driver sits on my rear bumper practically

Stick on my right hand signal as normal to turn into my street.

BMW accelerates past, with some guy practically sat on the NSR window as his chest was at roof level, turned left up this street cheering/shouting or something.

I'd have stopped and laughed if he fell out.

I vote plain stupid. Looks like a Skoda Favorit incident weekend.... http://briskoda.net/roadside-hotel/grrr-some-idiot-done-my-car-over/138412/

Probably some twoccers out for a joy ride or just your typical Beemer driver;);)

Ego and brain sized in inverse proportion to each other.