National Grid helicopter

Jan and I went for another local walk yesterday. As we left the house we could hear a helicopter nearby. The noise stayed fairly constant, indicating that it was probably hovering. Air Ambulance? Police? We walked towards the sound and saw that the helicopter was neither ambulance yellow nor police blue. It was this, a National Grid machine.

It seemed to be gradually working its way along a run of transmission lines. Was it surveying the lines themselves? Or the pylons?

Or was it surveying the land for a new set of power lines to connect to a new offshore windfarm? These transmission lines have only very recently been upgraded (five years ago), as I wrote about here.