D'Souza v DPP  UKHL 10
Under s17(1)(d) PACE 1984 a constable may enter and search any premises for the purpose of recapturing a person who is unlawfully at large and whom he is pursuing: (1) a detained patient who absconds is "unlawfully at large"; (2) the pursuit must be almost contemporaneous with the entry into he premises.
The applicant assaulted police officers in her own home. The police had gone there in the evening to retake his wife, who had absconded from hospital that afternoon. The propriety of convicting her depended on whether at the time the police were acting in the execution of their duty. That depended on whether they were entitled to enter by force without a warrant.
The applicant was lawfully detained in hospital and had gone absent without leave, so was "unlawfully at large" for the purposes of s17 PACE 1984. However, the police were not "pursuing" her as required by that section. The pursuit must be almost contemporaneous with the entry into he premises, but in this case the police had formed an intention to arrest when they discovered the facts and then gone to the premises.
The case was remitted to the Crown Court with directions to revoke the conviction.