One of the established defences to a claim in negligence is that of illegality. This defence has been tested in a number of cases in which a claimant has committed an offence whilst suffering from a psychiatric illness or condition. It has been tested again in before the Court of Appeal in Henderson v Dorset Healthcare University NHS Foundation Trust  EWCA Civ 1841.
The parties had agreed a statement of facts on the basis of which the judge at first instance, Jay J, had been asked to make a determination. He held that the claim should be dismissed for illegality. The Court of Appeal has upheld that decision. The facts as set out in the Court of Appeal's jdugment are as follows:
" Ms Henderson was born on 10 August 1971. She has been diagnosed at different times as suffering from paranoid schizophrenia or schizoaffective disorder. She began experiencing problems with her mental health in 1995. From about 2003 she had various formal (pursuant to the Mental Health Act 1983 ("the MHA 1983")) and informal hospital admissions. Her condition had recently worsened when, on 25 August 2010, whilst experiencing a serious psychotic episode, she stabbed her mother to death.
 At the time of the offence, Ms Henderson was under the care of the Southbourne community mental health team ("SCMHT"), which is managed and operated by the Trust. An independent investigation was commissioned by the NHS South West and the Bournemouth and Poole Adults Safeguarding Board. It found failings by the Trust in her care and treatment, ultimately concluding that, while the killing of Ms Henderson's mother could not have been predicted, a serious untoward incident of some kind was foreseeable based upon Ms Henderson's previous behaviour when experiencing a psychotic episode. The killing of Ms Henderson's mother was preventable and, had a rapid response been forthcoming, the tragic incident would probably not have occurred. It is, therefore, common ground between the parties that this tragic event would not have happened but for the Trust's breaches of duty in failing to respond in an appropriate way to Ms Henderson's mental collapse.
 Ms Henderson was charged with the murder of her mother. Having regard to the opinions of two consultant forensic psychiatrists, Dr Caroline Bradley and Dr Adrian Lord, the prosecution accepted a plea of manslaughter by reason of diminished responsibility."
A very similar case had been brought before the Court of Appeal in Clunis v Camden and Islington Health Authority  QB 978. In that case the court had dismissed the claim, holding that it was barred for illegality on grounds of public policy. A similar decision was reached in the negligence claim of Gray v Thames Trains Ltd  UKHL 33,  AC 1339.
Those decisions looked clear and determinative of the issue - Ms Henderson's claim was likely to be dismissed. Her argument was that those decisions were inconsistent with a more recent and more authoritative decision, in Patel v Mirza  UKSC 42,  AC 467. In Patel the issue was whether a claim to recover money paid to the defendant pursuant to an agreement for use of the money in betting on the movement of shares on the basis of inside information, contrary to the prohibition on insider dealing in section 52 of the Criminal Justice Act 1993, was barred by illegality. The Supreme Court held that it was not. Lord Toulson held at :
"The essential rationale of the illegality doctrine is that it would be contrary to the public interest to enforce a claim if to do so would be harmful to the integrity of the legal system (or, possibly, certain aspects of public morality, the boundaries of which have never been made entirely clear and which do not arise for consideration in this case). In assessing whether the public interest would be harmed in that way, it is necessary (a) to consider the underlying purpose of the prohibition which has been transgressed and whether that purpose will be enhanced by denial of the claim, (b) to consider any other relevant public policy on which the denial of the claim may have an impact and (c) to consider whether denial of the claim would be a proportionate response to the illegality, bearing in mind that punishment is a matter for the criminal courts. Within that framework, various factors may be relevant, but it would be a mistake to suggest that the court is free to decide a case in an undisciplined way. The public interest is best served by a principled and transparent assessment of the considerations identified, rather by than the application of a formal approach capable of producing results which may appear arbitrary, unjust or disproportionate."
The decision in Patel was in relation to a contract claim and although the Justices examined the principle of illegality more broadly, the Supreme Court did not expressly or implicitly overrule or doubt the decisions in Clunis and Gray. Thus the Court of Appeal in Henderson considered itself still bound by those precedents barring such claims, and has now added a third.