Britain's high-security jails have one area for prisoners who misbehave, and another for those deemed totally unmanageable.

The units, known as Close Supervision Centres (CSCs), are a step up from regular segregation wings – which you might be familiar with if you've ever spent an entire Sunday chain-watching Netflix documentaries about America's prisons – and have been described by HM Chief Inspector of Prisons as having the "most restrictive conditions with limited stimuli and human contact".

I exchanged letters with current inmate Graham, who has spent a considerable amount of time in these units, to find out what life is like in a CSC.

MY FELLOW INMATES

"The CSC unit is a hopeless, miserable place, filled with people who suffer from mental health issues. Many of them have been in secure mental hospitals. My neighbours are constantly shouting, screaming and breaking things, which really starts to affect you after a while. I sometimes try to pass the time by shouting to them through the cell window, but it's difficult to keep track of which voice belongs to which person without actually being able to see them.

"Some of the other residents are alright, but most are seriously disturbed. There are actually a fair few people in here who deliberately committed acts of violence to get sent to the CSC because they've got psychological problems and were unable to cope in mainstream prison-populations. They kick off whenever there's a chance of them being moved back to a normal wing so that they get to stay here. It's quite sad, really."

DAILY ROUTINE

"My days are filled with monotony, as there aren't many activities I'm actually allowed to do. Fortunately, I've got a TV in my cell, which helps to pass the time. That isn’t normal for people in the CSC, though. I'm lucky to have it, because there's fuck all else for me to do. I'm not allowed to use the library, and have limited access to books. I can have them sent in, but have to ask permission from the prison authorities, who might turn down the request depending on what book it is.

"In terms of physical activity, I'm given access to a cage each day to exercise in, but have to do so alone. This provides me with a slight change of scenery, but let's face it: there isn't a huge difference between being locked in a cell and being locked in a cage.

"I'm allowed a phone call and a shower each day, but am escorted everywhere by a group of seven guards. This is also dependent on there being no incidents elsewhere on the unit, because if the alarm goes off to indicate that another inmate is kicking off, my time out of the cell can be cut short. There have been periods in which there were incidents every single day for weeks on end, so this happens quite often."

WEAPONISING FAECES

"Even with so many security measures in place, prisoners in the CSC system can still find ways to attack each other. Inmates have to walk past the exercise cage while being escorted to the showers. Sometimes the prisoner in the cage will attempt to throw faeces at the passer-by. Inmates store their shit in bottles, ready to launch it at a rival when the opportunity arises. They sometimes also chuck piss out of the windows.

"Inmates have a lot of time on their hands, so they'll while away the hours by plotting and scheming, thinking of new ways to intimidate or attack their enemies. Some enjoy the fact that they can inflict harm on others while on the CSC without having to worry about getting their heads kicked in for it, because they're able to do it at a distance while surrounded by guards. This wouldn't be the case on a normal prison-wing; if you start something there, you have to deal with the consequences straight away."

COPING WITH ISOLATION

"The CSC is designed to break you. Most people are stuck here for the long haul with no hope of progression to another location. The powers-that-be are within their rights to keep prisoners here forever if they want to, which is a depressing thought. Some residents go insane from the vast periods of isolation. The constant noise and unpredictability of the environment undoubtedly contributes to their deterioration.

"I've known of inmates who pretended to be transgender in the hope that they'd be transferred to the female prison system, where there are no CSC units. It seems like a desperate measure, but these are desperate circumstances to be in. Some prisoners rebel against the system by flooding their cells, smearing shit all over the walls and starting fires in an attempt to get back at their captors.

"I prefer to write lots of letters and make use of my connection to the outside world as a mechanism for coping with the isolation. It's hard not to lose hope when you've been in here as long as I have, but I still dream of being moved back to a regular wing at some point in the future."