Or is it just down to the cop who writes the log and gives it 5 mins more or less each trip?
The TFOs have to account for every minute of the flight in the post flight log.
Do individual forces pay for travelling time to the job, or just from their force border?
They pay for travelling time, which can be a bit unfair at times. For example, the Hawarden cab can be on a job for Lancashire over Blackpool. They then get tasked for a job for Cheshire at Chester. Cheshire get billed for the 20 minutes or so that it takes to fly to Chester. However, if that job had come in 20 minutes later, just as the aircraft was about to land at Hawarden, Lancs would have paid for that transit time, and, Cheshire would only have had 2 or 3 minuets of transit time to pay for.