Hornsey, 29, will be locked up indefinitely under the Mental Health Act 1983 and will have to register as a sex offender, after admitting the indecent assault.
Alan Blake, prosecuting, told Oxford Crown Court yesterday the attack happened in the grounds of the Fair Mile Hospital, Cholsey, near Wallingford.
He said Hornsey asked the woman for directions then, without warning, grabbed her round the neck and pulled her to the ground.
He then committed a serious sexual assault before being pulled away by two maintenance workers. Dr Richard Noon, of the Oxford Clinic, Littlemore, said Hornsey was suffering from an acute form of schizophrenia.
He added: "There are a number of worrying features regarding this assault, first the severity, and secondly that Mr Hornsey said he would have continued if he had not been stopped."
Judge Kenneth Wilson-Mellor ordered that Hornsey should be detained in a secure unit in Thatcham, Berkshire.
Judge Wilson-Mellor added: "This offence was a very grave one and the effects must have been appalling."
Story date: Friday 04 February
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