R (P) v MHRT East Midlands and North East Region (2002) EWCA Civ 697
From Mental Health Law Online
Psychopathic disorder can persist for years without causing any abnormally aggressive or seriously irresponsible conduct; it is enough that the disorder of mind has done so in the past and that there is a real risk that, if treatment in hospital is discontinued, it will do so in the future.
Related case
See also R (P) v MHRT East Midlands and North East Region (2002) EWCA Civ 260 (permission to appeal to CA)
Update
A change made by the Mental Health Act 2007 is related to this case. See Appropriate treatment test replaces treatability test and applies to all patients under long-term detention 3/11/08