There is no legal obligation to stop and help if you are not involved in a crash for example. However, morally, paramedic's, nurses, doctors and people with more than just your average senior first aid are encouraged to stop and help. We are taught that the first response is vital, so anything that can be done in the first 5mins before an ambulance arrives could save someone's life.

That post above is quite an interesting read however. Should clear a few things up.