In Ratcliffe v McConnell in 1999, a claimant who had been drinking jumped into a shallow swimming pool during winter and suffered serious injuries.

The pool had been marked with warning signs that it was shallow. The claimant failed in his case against the owners of the property under what is known as the Occupier’s Liability Act.

The claimant was a adjudged to have accepted the risk and the owners of the pool avoided liability under the defence of volenti non fit injuria, which provides that no duty of care is owed to any person who willingly accepts the risk of what they are doing.

The judgement appears to make sense. Why should an occupier of land have responsibility for someone who behaves foolishly.

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But occupiers – home owners, councils, companies and institutions – do owe a duty of care to visitors to their premises.

This duty may extend to trespassers, which might seem odd. The case law developed after a series of accidents involving children trespassing on British Railway land.

The seminal case was British Railways Board v Herrington in 1972, when the House of Lords decided a landowner could owe a duty to trespassers on his land and the duty was to avoid injuring them.

The railway line operated by British Railways Board ran through property open to the public. Fences surrounding the railway were in disrepair and for many years children had been seen on the lines. The child was severely injured when he stepped on the line after entering through a broken fence and the House of Lords held that British Railways Board was in breach of its duty to avert the danger by fixing the fence. If the presence of trespassers is known, steps should have been taken.

These matters came to the fore in Young v Kent County Council. A 12-year-old was hurt falling through a fragile skylight on the roof of a school building run by the council. The claimant was at a youth club and tried to climb on to the roof using the flue of an extractor fan, as many had done before.

The claimant was a trespasser. He had to show that the council had breached a duty towards him.

The court held that the council should have known children were likely to climb the flue on to the roof. The area was a known meeting place for children. The judge accepted the boy was probably unaware of the fragility of the skylight, and ruled the council was in breach of the Occupiers Liability Act.

But an adult trespasser, he added, would have recovered nothing. The child was as much to blame as the council and the claim was halved by contributory negligence.

Contrast this with two actions involving children injured falling from trains after running beside a track and jumping on to them. The court held that a fence would not have deterred the claimants.

The volenti defence was successful; the children must have appreciated the risks of what they were doing. In the Young case, the boy knew it was wrong to climb on the roof but was unlikely to be aware of the state of the skylight.