This is the second Police Force the programme has followed, the first being Manchester.
Confusingly, the overall series title is more accurately "The Force:[Police Force]" - series 1 and 2 being Manchester, 3 and 4 Essex, etc.
Series 5 in the overall series was entitled "The Force: North East" - (series 1 of presumably 2, covering policing in that area in the North East of England). It appears that every 2 series cover a different Police Force area throughout the overall series run.