The leading case is McKew -v- Holland and Ors  : in that case a man’s leg had a tendency to give way without warning as a result of an injury for which the defendant admitted liability.
At a time when his claim was still being negotiated, he went alone at a council flat down a long flight of steep stairs with no hand rail. His leg gave way and he broke his ankle, making the original injury far worse.
The Law Lords held that a test was whether he had acted reasonably and carefully bearing in mind his knowledge about his injury. They found that his unreasonable conduct had broken the chain of causation for which the original negligent employer should be liable.
So the test is one of reasonableness of action.
In this case the negligent employer had caused the claimant a knee injury. Three years later, as a consequence of the injury, the claimant had an above-knee amputation. He then started to wear a prosthetic limb.
He could not wear his prosthesis while driving so would place it on the back seat. Because it was difficult to put the prosthesis on and off, he would instead use a pair of sticks outside the car for short journeys.
On this occasion he was filling his car with petrol at a pump. He did not ask the Sainsbury’s petrol attendant to help him. Nor did he use his prosthesis or his sticks but instead steadied himself against the car and filled his tank.
As he went back to his driver’s door, from where he intended to call a cashier to take his payment, he caught his foot against a slightly raised manhole cover and fell. This caused him significant injury which was to confine him to a wheelchair for the rest of his life.
There was no case arising against Sainsbury’s for the manhole cover. The question here was whether the original negligent defendant should pay for this injury four years later. The claim was still being negotiated at that time.
It was agreed by all parties that, if the original employer were liable, there was one-third contributory negligence by the claimant in failing to see the obstruction that caused the trip and in failing to use his sticks. That deduction for contributory negligence was approved by Lady Justice Smith.
The defendant wished the Law Lords to apply a test of recklessness but they were not attracted to language that spoke of a criminal law test and said the test was one of reasonableness.
They noted that, when McKew was applied in Emeh -v- Kensington etc AHA  3ORER 1044 Waller LJ said: “The degree of unreasonable conduct which is required (to break the chain of causation) is … very high.”
At the first instance the Judge said that the claimant was “carrying out an every day task which he had done a number of times before without incident. He was seeking to act without reliance on others; in general terms his determination to live his life as normally as possible is to be commended”.
His conduct was not what could be described as “McKew unreasonable”.
The Judge said: “The way he went about the car should be taken into account (only) as a matter of contributory negligence.”
The Court of Appeal completely approved this approach.
The appeal was dismissed and the original defendant was liable for the consequences of this injury subject to a one-third deduction for the Claimant’s part in the accident .
Spencer v Wincanton Logistics, CA.