Put a violent criminal in prison and most people accept that he will eventually be released. Put a violent criminal into secure psychiatric care and there is a widespread view that he should stay there for ever, even though responsibility for his actions may lie, in whole or in part, with his illness.

Offenders with, for instance, paranoid schizophrenia and personality disorders are more frightening to the general public than ordinary career criminals. They commit their crimes unexpectedly and for irrational reasons. That is why the doctors and hospital staff in the secure unit at the Bethlem Royal Hospital in south London, whom we met in a compelling documentary called Life on the Psych Ward (29 November), have a difficult job.

Their task is to help 120 “mentally disordered offenders” – among them murderers and rapists – prepare themselves for the possibility of eventual return to the outside world. They do not just lock people up; they provide therapy.

The public often assert that secure psychiatric care is a soft option compared with prison, and certainly the physical conditions and daily routine are better. There is one big difference, however: prison sentences usually come with an end point but detention in a secure hospital can last for ever.