balance of risk, gravity of injury, cost and social value.
My Lords, the majority of the Court of Appeal appear to have proceeded on the basis that if there was a foreseeable risk of serious injury, the Council was under a duty to do what was necessary to prevent it. But this in my opinion is an oversimplification. Even in the case of the duty owed to a lawful visitor under section 2(2) of the 1957 Act and even if the risk had been attributable to the state of the premises rather than the acts of Mr Tomlinson, the question of what amounts to "such care as in all the circumstances of the case is reasonable" depends upon assessing, as in the case of common law negligence, not only the likelihood that someone may be injured and the seriousness of the injury which may occur, but also the social value of the activity which gives rise to the risk and the cost of preventative measures. These factors have to be balanced against each other.
For example, in Overseas Tankship (UK) Ltd v Miller Steamship Pty Ltd (The Wagon Mound (No. 2))  1 AC 617 , there was no social value or cost saving in the D's activity. Lord Reid said (at p 643): " . In the present case there was no justification whatever for discharging the oil into Sydney Harbour. Not only was it an offence to do so, but it involved considerable loss financially. If the ship's engineer had thought about the matter, there could have been no question of balancing the advantages and disadvantages. From every point of view it was both his duty and his interest to stop the discharge immediately." .
So the Ds were held liable for damage which was only a very remote possibility. Similarly in Jolley v Sutton London B.C.  1 WLR 1082 there was no social value or cost saving to the Council in creating a risk by leaving a derelict boat lying about. It was something which they ought to have removed whether it created a risk of injury or not. So they were held liable for an injury which, though foreseeable, was not particularly likely. On the other hand, in The Wagon Mound (No. 2) Lord Reid (at p. 642) drew a contrast with Bolton v Stone  AC 850 in which the HL held that it was not N for a cricket club to do nothing about the risk of someone being injured by a cricket ball hit out of the ground.