Brian Barder Blog which sets out the iniquities of the system of imprisonment known as Indeterminate Sentence for Public Protection (or IPP).
This form of "indeterminate sentence" was introduced by the Criminal Justice Act 2003 s.225 (Imprisonment for Public Protection - for adults) and s.226 (detention for Public Protection - for those under age 18). The law has since been modified by the Criminal Justice and Immigration Act 2008 s.47. The law is of labyrinthine complexity - see Wikicrimeline - and numerous (and expensive) appellate decisions have resulted and continue to do so.
The essential point is that the prisoner is not automatically released even when a public protection tariff period has been served. It then becomes a matter for the Parole Board which must be satisfied that, if released, the prisoner would not pose an unacceptable risk to the public. Due to underfunding of relevant rehabilitative courses, this can be impossible to assess and so the prisoner remains incarerated until his next review which might be months away. This has, in turn, obviously swollen considerably the numbers in prison since people are being held long after they have actually served their tariff. 1 in 15 prisoners is now an an IPP prisoner with the consequential enormous costs.
The government is undertaking a review of sentencing. Reform in this area is essential - see Prison Reform Trust March 2010
Addendum 30th June: The Independent reported "Paedophile dubbed "every parent's nightmare" jailed indefinitely.