David Hurst is a agent of the British Special Branch.
Working out of the Greater Manchester Police's Integrated Special Branch Office, Hurst has been with the department for some time, enough to hold the rank of Detective Sergeant. During his early days with the department, he was occupied with fighting the IRA and its various splinter groups. Now that those hostilities had for the most part died down, it was Al Qaeda that was his focus.
Hurst was a product of one of the poorest areas of Liverpool and though he and his non-identical twin brother, Peter, made it out fine, the rough-and-tumble upbringing that the neighborhood forced upon him has served him well as a member of the police force. Knowing a lot about the thoughts and desires of miscreants because he so frequently walked the line of legality as a kid meant that he had a leg up on many of his colleagues when it came to knowing what might or might not happen next.
Being a part of Special Branch, it is not uncommon for Hurst to be forced to interact with MI-5 though it is usually with considerable annoyance. As he mentions more than once to his fellow officers, that organization is forthcoming with 95% of whatever valuable intel was available but it seemed inevitable that not knowing the remaining 5% gets someone seriously dead.
This is why when in the first recorded adventure he is ordered to work with the new liaison officer, Debbie Heron, he was less than pleased. That lack of interest will changed considerable as they get to know each other.