On a freezing, rainy day in December 2017, I arrive at the gate of Her Majesty's Prison Brixton in London. Having travelled from the Australian summer, the imposing, grimy brick walls of the 200-year-old prison appear especially bleak against the grey sky. Although it has been three years since my last visit, the gate officer remembers and greets me as I complete the fingerprint and face recognition security checks. I pass through the airlock to meet Andrew Wilkie, PRA Director of Radio and Operations, and it starts to snow as we step outside to the courtyard. He unlocks and locks the multiple doors and high cage-like metal gates for us to cross to the building where National Prison Radio (NPR) occupies a small set of rooms at the end of a narrow windowless corridor.

It is ten years since my first visit to the prison. At that time, the space was a dark dingy storeroom with walls covered in peeling paint and mould. Today, I walk into a bright, bustling creative space, with 15 NPR staff and prisoners working together. The two studios take up the left hand side of the room, NPR staff are busy at computer workstations against the back wall, and the central production area is filled with a new group of prisoners, all editing audio, writing, and talking through ideas with an NPR producer.

Since the 2007 launch of Electric Radio Brixton, the space has evolved into a national radio hub, broadcasting content produced in prisons around the country. Year after year, more people are listening to National Prison Radio, listening for longer, writing in, and getting involved. Reflecting on the distance travelled, PRA Chief Executive Phil Maguire (2016) believes that the service is “more important than it's ever been”. He describes a constant feedback loop with listeners and partners, helping the organisation to continually improve, innovate, and understand the needs of the audience.

The PRA evaluation strategy is central to the process. Developed with support from the BBC Radio Audiences Team, ImpetusPEF and Radio Joint Audience Research (RAJAR), the official body in charge of measuring radio audiences in the UK, the methodology reflects broader radio industry practice.