Harry Roberts is serving a triple Life Sentence for killing three Policemen in 1966.

In 1988 Harry Roberts was in his 28th year of prison, serving in Gartree Prison.

According to The BBC - Crime Fighters - Prison Services website, Adult Male Prisoners are classified under 4 security categories, A, B, C and D. Each of these Categories define the severity of all the inmates' crimes and the risk the prisoner would pose to society/public should they escape.

Category A prisoners are the most dangerous offenders who would pose the highest risk to society should they escape.
Category B offenders are still dangerous enough to warrant high security, yet would pose a less of a threat to society in the event of an escape.
Category C prisoners are considered to lack the desire and/or skills to escape, therefore posing a less risk to the public.
Category D prisoners are the remaining offenders who do not pose a risk to society and are unlikely to escape.

Harry Roberts spent over 20 years as a Category A prisoner, breaking the record of 20 years.

An example of the tight security on Category A inmates can be seen in the visitation rules. If someone wants to visit a Category A prisoner, the prisoner must first ask their wing officer for an application form so the visitor can be added to their already approved list of visitors. The visitor must fill in their personal credentials and if not a relative state how and when they came to know the prisoner. The Governor then will write to the visitor asking if they want to visit and request two passport sized photographs to be sent to the prison. Upon receipt of the photos, the prison will send someone to check if the photos are indeed the intended visitor. After all of that, when the visit(s) occur they are in a seperate room, in the presence of two officers.

References: Kray, Kate Lifers, Blake Publishing Ltd. (1996).
BBC - Crime Fighters - Prison Services, http://www.bbc.co.uk/crime/fighters/prisonservice.shtml.