Doubt you'd spot anything minor happening behind you really, but a bigger accident I'd probably end up stopping, if just to contact the police so they can put the signs on.
With regards not parking etc. - it is an offence normally yes, but you can use the hard shoulder in an emergency (not sure about reversing). Infact the point often made about not staying in your car when you break down would be inciting an illegal act if you couldn't walk on a motorway.
Just follow the number 1 rule of accidents - make the scene safe before attending to anyone!
Trev