“A duty to protect against obvious risks or self-inflicted harm exists only in cases where in which there is no genuine and informed choice, as in the case of employees whose work requires them to take the risk, or some lack of capacity, such as the inability of children to recognise danger (Herrington v British Railways Board[1972] AC 877 ) or the despair of prisoners which may lead them to inflict injury on themselves: Reeves v Commissioner of Police of the Metropolis[2000] 1 AC 360 .”